Heart pump

ABSTRACT

A heart pump including: a housing forming a cavity including: at least one inlet aligned with an axis of the cavity; and, at least one outlet provided in a circumferential outer wall of the cavity; an impeller provided within the cavity, the impeller including vanes for urging fluid from the inlet to the outlet; and, a drive for rotating the impeller in the cavity and wherein a flow path through the pump has a minimal cross-sectional area of at least 50 mm 2 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 120 toInternational Patent Application No. PCT/US2017/012503 entitled “HeartPump,” filed on Jan. 6, 2017 which claims priority under 35 U.S.C. § 119to U.S. Provisional Patent Application No. 62/275,754 entitled “HeartPump,” filed on Jan. 6, 2016; U.S. Provisional Patent Application No.62/275,723 entitled “Heart Pump Impeller Rotational Speed Control,”filed on Jan. 6, 2016; and U.S. Provisional Patent Application No.62/275,744 entitled “Heart Pump With Impeller Axial Position Control,”filed on Jan. 6, 2016, the contents of all of which are incorporatedherein by reference in their entireties.

BACKGROUND OF THE INVENTION

The present invention relates to a heart pump and in particular to aheart pump including improved flow characteristics.

DESCRIPTION OF THE PRIOR ART

The reference in this specification to any prior publication (orinformation derived from it), or to any matter which is known, is not,and should not be taken as an acknowledgment or admission or any form ofsuggestion that the prior publication (or information derived from it)or known matter forms part of the common general knowledge in the fieldof endeavour to which this specification relates.

The use of rotary impeller based mechanical pumps to treat heart failureis increasing as the general population ages and the number of donororgans for heart transplantation remains limited. Devices can be used tobridge a patient to heart transplant, to recovery, or indeed as adestination alternative. They can be configured to assist the heart inits function, or replace it entirely.

WO2004098677 and WO2006053384A1 each describe a double sided impellerthat rotates at a common speed, with each side of the impellerrespectively configured for left and right heart support. Thiseffectively introduces an inherent problem regarding the ability toindependently control and thus balance the outflow from the left andright sides of the device, i.e. an increase in impeller rotational speedwith produce a corresponding increase in outflow from both cavities.

WO2006053384A1 addressed this issue by introducing the ability toaxially displace the rotating impeller within the cavity so as tosimultaneously alter the relative efficiencies of each side of thedevice. However, when the control method used to achieve this axialdisplacement is active, such pumps require the use of feedback signalsfrom pressure sensors and the like to actively control and maintain adesired set axial location. This method of control would inherentlyconsume excessive amounts of electrical power and introduce issuesrelating to the long term reliability of blood contacting sensors.

U.S. Pat. No. 8,636,638 describes a controller for a heart pump thatdetermines movement of an impeller within a cavity in a first axialdirection, the cavity including at least one inlet and at least oneoutlet, and the impeller including vanes for urging fluid from the inletto the outlet, causing a magnetic bearing to move the impeller in asecond axial direction opposite the first axial direction, the magneticbearing including at least one coil for controlling an axial position ofthe impeller within the cavity, determining an indicator indicative ofthe power used by the magnetic bearing and causing the magnetic bearingto control the axial position of the impeller in accordance with theindicator to thereby control a fluid flow between the inlet and theoutlet.

U.S. Pat. No. 7,435,059 describes a system for pumping blood to assistor assume the cardiac function of a patient is characterized by a bloodpump that exhibits a steep pump curve such that only small changes inpump flow occur for large changes in differential pressure across thepump. The pump therefore exhibits flow-limiting characteristics toprotect the physiological system against harmful flow rates. Pump flowmay also be limited by controlling the current provided to a driver froma power supply or by suitable restrictions within or external to thepump housing.

The natural heart continuously balances systemic and pulmonary flowthrough ventricular interdependence and the Frank-Starling mechanism.This mechanism relates directly to the pressure sensitivity of theheart, and is an important feature to enable the heart to balance leftand right blood flow and thus pressure in the face of alterations inpatient state, such as postural changes, coughing, straining/Valsalvaand transitions to exercise. It also effectively accommodates for thenatural flow imbalance created by the bronchial circulation, whichshunts blood from the aorta back to the pulmonary venous network. Whenthe heart fails, the Frank-Starling mechanism and thus this pressuresensitivity is compromised.

Mechanical circulatory support is an effective means to restore bloodflow to the circulatory system in the case of a failing heart, howevercurrent blood pumps do not exhibit the same pressure sensitivity as thenative or even failing heart. This support can be in the form of aventricular assistance device (VAD), whereby the device assist thefunction of one or more of the failing ventricles, or completereplacement of the ventricles with a total artificial heart (TAH).

Reduced pressure sensitivity results in the potential for flow and thuspressure imbalance in the systemic and pulmonary circulations. Highinflow (venous) pressures caused by a hypo functioning side of theheart/device can result in the accumulation of fluid in interstitialspaces (oedema). Should this occur in the pulmonary venous network,efficient gas exchange in the lungs may be impacted. When it occurs inthe systemic venous network, liver and kidney failure can result.

Low inflow (venous) pressures resulting from a hyper functioning side ofthe heart/device can lead to vessel wall collapse, leading to anincrease in resistance for blood to enter the device, and ultimately areduction or indeed cessation of forward flow from the device.Conversely, a hyper functioning pump may also raise arterial pressure tolevels that may predispose the patient to haemorrhagic stroke or otherhypertensive related complications.

In the case of single ventricular assistance (VAD), usually left sided(LVAD), the native ventricles remains with varying degree ofFrank-Starling mechanism intact and thus varying degrees of pressuresensitivity remain in the left and right ventricle, from normal tocompromised. A normal right heart can assist with flow balance, howevera more pressure sensitive left device assisting the left heart willresult in greater changes in device outflow for changes in patientstate, and thus both a higher sustained cardiac output during exerciseand faster return to flow balance, mitigating the requirement forchanges in device rotational speed.

In instances whereby the two failing ventricles are completely removedfor the implantation of a total artificial heart (TAH), the deviceitself must reproduce the Frank-Starling mechanism and thus pressuresensitivity and ultimately correct imbalances in flow.

Traditional wisdom has been to produce a pump that has optimum operatingefficiency at a typical flow rate for a subject, which typicallycorresponds to a flow rate of about five to six litres per minute,thereby minimising the power consumed by the pump. Additionally, it istypical to design pumps having a relatively low flow sensitivity topreload, as described for example in U.S. Pat. No. 7,435,059, so thatthe pump exhibits flow-limiting characteristics to protect thephysiological system against harmful flow rates or pressures.

Such configurations result in a heart pump having a steep pump curvewhich is a plot of the flow rate against head pressure across the pump(the difference between the inlet and outlet pressures), for a givenimpeller rotational speed. This shows that a large change in pressuresis required in order to cause a change in flow rate through the pumpthereby providing the aforementioned flow-limiting characteristics.

In such an arrangement, it may be necessary to vary the rotational speedof the impeller to thereby control pump outflow, in order to account forchanges in pressures within the subject's circulatory system. However,such control systems may require information regarding the physiologicalstate of the subject, such as blood pressures or flow rates, in order tofunction correctly. This requires the use of complex sensing techniquesand/or implanted sensors, which are undesirable, and in many cases makesassumptions regarding at least some parameters, such as blood viscosity,meaning they can be inaccurate. As a result, many existing heart pumpshave only a limited ability to accommodate physiological changes,meaning the subjects are often restricted in terms of activities theyare able to perform.

Flow path areas tend to influence the resistance of fluid to flowthrough the pump and thus outflow pressure sensitivity, with largerareas generally considered to create flatter pump curves.

However larger areas also tend to assist with device biocompatibility.Larger areas between the rotating impeller and the stationary casingresult in lower shear stresses levels in this region and thus reducedred blood cell lysis (haemolysis). However larger flow path areas arealso suggested to reduce the incidence of vonWillebrand factor (vWF)degradation. Degradation of this molecule can lead to impaired abilityfor blood to clot, thus raising the risk of bleeding complications.

Finally, another major complication with currently implanted rotaryblood pumps is device failure due to pump thrombosis. A clot eitherformed in low flow regions within the device, or formed in thecirculatory system and ingested into the device, may lodge in theimpeller blades or small clearances around the rotating impeller,causing the device to stop. These small clearances are often found atthe site of a contact bearing or hydrodynamic bearing. Thus, selectingimpeller blades that have large separations between them, andimplementing a maglev system that widens the surrounding clearances, mayreduce the incidence of pump thrombosis. This has particular relevanceto the right circulation, whereby emboli originating from the deep veinsmay enter into the right sided impeller. Hence large clearances andlarge flow paths may assist in allowing such emboli to pass unimpededthrough the device, to be filtered by the small capillaries in thepulmonary network.

SUMMARY OF THE PRESENT INVENTION

In one broad form an aspect of the present invention seeks to provide aheart pump including: a housing forming a cavity including: at least oneinlet aligned with an axis of the cavity; and, at least one outletprovided in a circumferential outer wall of the cavity; an impellerprovided within the cavity, the impeller including vanes for urgingfluid from the inlet to the outlet; and, a drive for rotating theimpeller in the cavity and wherein a flow path through the pump has aminimal cross-sectional area of at least 50 mm².

In one embodiment a flow path through the heart pump has across-sectional area throughout the entire pump that is at least one of:at least 75 mm²; at least 100 mm²; at least 125 mm²; at least 140 mm²;at least 150 mm²; at least 200 mm²; and, up to 300 mm².

In one embodiment the pump has a performance curve having a gradient ofless than −20% over a defined flow range such that a change in pressureof 10 mmHg across the pump causes a change in flow rate of at least 2LPM, the defined flow range being between at least one of 5 LPM to 8LPM; 3 LPM to 12 LPM; and, 3 LPM to 15 LPM.

In one embodiment the pump generates a pressure head that is at leastone of: for a pump that provides at least partial left ventricularfunction: between 60 mmHg and 100 mmHg at 6 LPM; between 70 mmHg and 90mmHg at 6 LPM; and, approximately 80 mmHg at 6 LPM; and, for a pump thatprovides at least partial right ventricular function: between 10 mmHgand 30 mmHg at 6 LPM; between 15 mmHg and 25 mmHg at 6 LPM; and,approximately 20 mmHg at 6 LPM.

In one embodiment the heart pump provides at least partial leftventricular function.

In one embodiment the heart pump has a pump performance curve having agradient less than at least one of: −25%; −30%; −35%; −40%; −100% −200%;and, −500%.

In one embodiment an axial position of the impeller within the cavitycontrols in part a flow of fluid from the inlet to the outlet, andwherein a change in axial position of 200 μm causes at least one of: achange in flow rate of at least one of: at least 1 LPM; at least 2 LPM;less than 4 LPM; and, between 2 LPM and 3 LPM; and, a change in flowpressure of at least one of: at least 5 mmHg; at least 10 mmHg; at least15 mmHg; at least 20 mmHg; at least 25 mmHg; at least 30 mmHg; at least35 mmHg; and, at least 40 mmHg.

In one embodiment the outlet at least one of: has a throat area of atleast one of: at least 60 mm²; at least 80 mm²; at least 120 mm²;between 60 mm² and 250 mm²; between 120 mm² and 160 mm²; between 140 mm²and 160 mm²; between 140 mm² and 250 mm²; between 130 mm² and 150 mm²;approximately 140 mm²; and, approximately 150 mm²; has a substantiallyrectangular cross-sectional shape and a width to height aspect ratio ofat least one of: between 1:2 and 2:1; between 1:1 and 2:1; between 1:1and 1.8:1; between 1.1:1 and 1.6:1; and, approximately 1.4:1; and,defines a cutwater angle of at least one of: between 0° and 70°; between30° and 50°; between 40° and 45°; between 35° and 45°; between 45° and50°; between 0° and 60°; and approximately 45°.

In one embodiment the impeller has at least one of: a vane height of atleast one of: at least 1.5 mm; less than 5 mm; between 1.5 mm and 3 mm;between 1.7 mm and 2.3 mm; between 1.8 mm and 2.2 mm; and, between 1.9mm and 2.1 mm; and, approximately 2 mm; and, a vane inlet angle of atleast one of: less than 90°; greater than 60°; between 70° and 90°;between 82° and 86°; and, approximately 84°; and, a vane outlet angle ofat least one of: less than 60°; greater than 20°; between 30° and 50°;between 35° and 45°; between 38° and 42°; and, approximately 40°.

In one embodiment the impeller includes at least one of: a number ofprimary vanes, the primary vanes having an inner diameter of at leastone of: larger than a diameter of an inflow port; at least 10 mm; lessthan 40 mm; between 20 mm and 40 mm; between 25 mm and 35 mm; and,approximately 25-30 mm; a number of secondary vanes, the secondary vaneshaving an inner diameter of at least one of: at least 20 mm; less than40 mm; between 30 mm and 40 mm; and, approximately 35 mm; and, an outervane diameter of at least one of: at least 20 mm; less than 60 mm;between 45 mm and 55 mm; between 48 mm and 52 mm; and, approximately 50mm.

In one embodiment the primary vanes having an outer thickness of atleast one of: at least 5 mm; less than 20 mm; between 6 mm and 15 mm;between 7 mm and 8 mm; and, approximately 7.5 mm.

In one embodiment the impeller includes at least one of: an equal numberof primary and secondary vanes; at least three primary and secondaryvanes; less than six primary and secondary vanes; and, four primary andfour secondary vanes.

In one embodiment in a region of an outlet volute the cavity has atleast one of: a base circle diameter of at least one of: at least 40 mm;at least 50 mm; less than 100 mm; less than 80 mm; between 50 mm and 74mm; between 54 mm and 64 mm; and, approximately 60 mm; and, an outerwall diameter of at least one of: at least 50 mm; less than 100 mm; lessthan 80 mm; between 50 mm and 80 mm; between 65 mm and 76 mm; and,approximately 71 mm.

In one embodiment the housing includes a split volute.

In one embodiment over the defined flow range the volute generates amaximum radial force of less than at least one of 1.2N, 1.0N and 0.85N,and wherein the defined flow range is at least one of: 5 LPM to 8 LPM; 3LPM to 12 LPM; and, 3 LPM to 15 LPM.

In one embodiment the heart pump provides at least partial rightventricular function.

In one embodiment the heart pump has a pump performance curve having agradient less than at least one of: −30%; −35%; −40%; −75%; −100%; and,−150%.

In one embodiment an axial position of the impeller within the cavitycontrols in part a flow of fluid from the inlet to the outlet, andwherein a change in axial position of 200 μm causes at least one of: achange in flow rate of at least one of: at least 0.2 LPM; at least 0.5LPM; less than 2 LPM; and, between 0.5 LPM and 1.5 LPM; and, a change inflow pressure of at least one of: at least 1 mmHg; at least 2 mmHg; atleast 5 mmHg; and, at least 10 mmHg.

In one embodiment the outlet at least one of: has a throat area of atleast one of: at least 100 mm²; at least 130 mm²; between 130 mm² and250 mm²; between 130 mm² and 230 mm²; between 170 mm² and 210 mm²;between 140 mm² and 200 mm²; between 140 mm² and 210 mm²; between 150mm² and 200 mm². approximately 233 mm²; approximately 175 mm²; and,approximately 150 mm²; has a substantially rectangular cross-sectionalshape outlet and a width to height aspect ratio of at least one of:between 1:3 and 1:1; and, approximately 0.45-0.65:1; and, a width ofbetween 8 mm and 12 mm; and, defines a cutwater angle that is at leastone of: between 90° and 180°; between 90° and 135°; between 0° and 90°;between 45° and 90°; between 45° and 135°; between 60° and 80°; andapproximately 70°.

In one embodiment the impeller has at least one of: a vane height of atleast one of: at least 10 mm; less than 30 mm; between 10 mm and 25 mm;between 15 mm and 20 mm; between 17 mm and 18 mm; and, approximately17.5 mm; a vane inlet angle of at least one of: greater than 60°; lessthan 115°; between 80° and 100°; and, approximately 90°; and, a vaneoutlet angle of at least one of: greater than 60°; less than 115°;between 80° and 100°; approximately 72° and, approximately 90°.

In one embodiment the impeller includes: a number of primary vanes, theprimary vanes having an inner diameter of at least one of: at least 10mm; less than 25 mm; between 10 mm and 20 mm; between 14 mm and 18 mm;and, 16 mm; a number of secondary vanes, the secondary vanes having aninner diameter of at least one of: at least 10 mm; less than 25 mm;between 15 mm and 25 mm; between 18 mm and 20 mm; and, approximately 19mm; and, an outer vane diameter of at least one of: at least 15 mm; lessthan 40 mm; between 20 mm and 30 mm; between 22 mm and 27 mm; and,approximately 24 mm; and approximately 25 mm.

In one embodiment the primary vanes at least one of: have a thickness ofat least one of: at least 0.5 mm; less than 3.0 mm; between 0.75 mm and2.5 mm; and, 1.5 mm; and, have a filleted edge of between 0.25 mm and1.14 mm.

In one embodiment the impeller includes at least one of: an equal numberof primary and secondary vanes; between three and five primary vanes;four primary vanes; between three and six secondary vanes; foursecondary vanes; and, four primary vanes and four secondary vanes.

In one embodiment the inlet has a diameter of at least one of: at least10 mm; at least 15 mm; less than 30 mm; less than 25 mm; between 18 mmand 22 mm; and, approximately 19 mm to 20 mm.

In one embodiment the cavity has a diameter of a least one of: at least20 mm; at least 25 mm; less than 40 mm; less than 30 mm; between 27 mmand 29 mm; and, approximately 28 mm.

In one embodiment the impeller includes a rotor having a height of atleast one of: at least 5 mm; less than 15 mm; between 6 mm and 13 mm;and, between 8 mm and 11 mm; and, approximately 10 mm.

In one embodiment the rotor has an outer circumferential wall spacedfrom an inner cavity wall by at least one of: an average distance of atleast 2 mm; an average distance of less than 8 mm; an average distanceof less than 5 mm; and, an average distance of approximately 4 mm.

In one embodiment the impeller includes first and second sets of vanesprovided on a rotor body, the rotor being positioned within the cavityto define: a first cavity portion having a first inlet and a firstoutlet, the first set of vanes being provided within the first cavityportion so as to define a first pump that provides at least partial leftventricular function; and, a second cavity portion having a second inletand a second outlet, the second set of vanes being provided within thesecond cavity portion so as to define a second pump that provides atleast partial right ventricular function.

In one embodiment the heart pump is a total artificial heart.

In one embodiment the axial position of the impeller determines aseparation between each set of vanes and a respective housing surface,the separation being used to control the fluid flows from the inlets tothe outlets.

In one embodiment the first and second pumps have respective pumpperformance curve having different gradients so that a change inrotational speed of the pump causes a change in the relative flows ofthe first and second pumps.

In one embodiment the first and second pumps have a design pressureratio at 6 LPM of 3.5-4.5:1.

In one embodiment the first and second pumps have an axial pressuresensitivity of at least one of: at least 20 mmHg/mm; and, approximately60 mmHg/mm.

In one embodiment the first and second pumps have a change in designpressure ratio at 6 LPM of 3.25-4.75:1.

In one embodiment the drive includes: a number of circumferentiallyspaced permanent magnets mounted in the rotor of the impeller, adjacentmagnets having opposing polarities; and, at least one drive coil that inuse generates a magnetic field that cooperates with the magneticmaterial allowing the impeller to be rotated.

In one embodiment the housing and impeller cooperate to provide a shuntflow path between the first and second cavity portions, the shunt flowpath having a cross-sectional area that is at least one of: at least 15mm²; no greater than 50 mm²; between 20-50 mm²; and, approximately 25mm².

In one embodiment the shunt flow path cross-sectional area is adjustableby controlling an axial position of the impeller within the cavity.

In one embodiment the pump includes a magnetic bearing for controllingan axial position of the impeller within the cavity.

In one embodiment the magnetic bearing includes: first and secondannular magnetic bearing members mounted within and proximate a face ofthe rotor, the first magnetic bearing member being outwardly of thesecond magnetic bearing member; a number of circumferentially spacedsubstantially U-shaped bearing stators mounted in the housing proximatea second end of the cavity, each U-shaped bearing stator having firstand second bearing stator legs substantially radially aligned with thefirst and second magnetic bearing members respectively; and, at leastone bearing coil on each bearing stator that generates a magnetic fieldthat cooperates with the magnetic bearing members to thereby at leastone of: control an axial position of the impeller; and, at leastpartially restrain radial movement of the impeller.

In one embodiment the drive is positioned at a first end of the cavityand the magnetic bearing is positioned at a second end of the cavity.

In one broad form an aspect of the present invention seeks to provide aheart pump including: a housing forming a cavity including: a firstcavity portion having a first inlet aligned with an axis of the firstcavity portion and a first outlet provided in a circumferential outerwall of the first cavity portion; and, a second cavity portion having asecond inlet aligned with an axis of the second cavity portion and asecond outlet provided in a circumferential outer wall of the secondcavity portion; and, an impeller provided within the cavity, theimpeller including: a first set of vanes being provided within the firstcavity portion so as to define a first pump that provides at leastpartial left ventricular function; and, a second set of vanes beingprovided within the second cavity portion so as to define a second pumpthat provides at least partial right ventricular function; and, a drivefor rotating the impeller in the cavity and wherein the first and secondpumps define a respective flow path through the pump, each flow pathhaving a minimal cross-sectional area of at least 50 mm².

In one embodiment the first and second pumps have at least one of: adesign pressure ratio at 6 LPM of 3.5-4.5:1. an axial pressuresensitivity of at least one of: at least 20 mmHg/mm; and, approximately60 mmHg/mm.

In one broad form an aspect of the present invention seeks to provide aheart pump including: a housing forming a cavity including: at least oneinlet aligned with an axis of the cavity; and, at least one outletprovided in a circumferential outer wall of the housing; an impellerprovided within the cavity, the impeller including vanes for urgingfluid from the inlet radially outwardly to the outlet; and, a drive forrotating the impeller in the cavity and wherein the pump includes atleast one of: improved outflow pressure sensitivity (OPS); improvedaxial pressure sensitivity (APS); reduced radial hydraulic forces;emboli tolerance; and, improved passive flow balancing.

In one broad form an aspect of the present invention seeks to provide aheart pump including: a housing forming a cavity including: at least oneinlet aligned with an axis of the cavity; and, at least one outletprovided in a circumferential outer wall of the cavity; an impellerprovided within the cavity, the impeller including vanes for urgingfluid from the inlet to the outlet; and, a drive for rotating theimpeller in the cavity and wherein the pump has a performance curvehaving a gradient of less than −20% over a defined flow range such thata change in pressure of 10 mmHg across the pump causes a change in flowrate of at least 2 LPM, the defined flow range being between at leastone of 5 LPM to 8 LPM; 3 LPM to 12 LPM; and, 3 LPM to 15 LPM.

In one broad form an aspect of the present invention seeks to provide amethod of operating a biventricular heart pump during implantation, themethod including: connecting the heart pump to the pulmonary andsystemic circulatory systems; blocking a left pump outflow to the systemcirculatory system; operating the pump so that blood flow recirculatesthrough the lungs with blood flow received from the lungs via a leftpump inlet being shunted to a right pump via a left/right shunt flowpath so that blood is supplied to the lungs via a right pump outlet;once the lungs are perfused, unblocking the left pump outflow so thatblood flows through the pulmonary and systemic circulatory systems.

In one embodiment the method includes: initially operating the pump atat least one of: a rotational speed of between 1000 RPM and 1250 RPM;and, a blood flow rate though the lungs is approximately 0.5 LPM; and,increasing the rotational speed of the pump until at least one of: therotational speed is approximately 1800 RPM; and, the blood flow ratethough the lungs increases to approximately 1.5-2 LPM.

In one embodiment the method includes: pumping blood through the lungsuntil at least one of: for between 5 minutes and 10 minutes; and, untillung resistance reduces; and, unblocking the left pump outflow to thesystem circulatory system.

It will be appreciated that the broad forms of the invention and theirrespective features can be used in conjunction and/or independently, andreference to separate broad forms in not intended to be limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

Various examples and embodiments of the present invention will now bedescribed with reference to the accompanying drawings, in which:—

FIG. 1A is a schematic perspective view of an example of a heart pump;

FIG. 1B is a schematic cutaway view of the heart pump of FIG. 1A;

FIG. 1C is a schematic perspective exploded view of the heart pump ofFIG. 1A;

FIG. 1D is a schematic diagram of an example of a control system for theheart pump of FIG. 1A;

FIG. 2A is a schematic perspective top side view of an example of adrive magnet configuration;

FIG. 2B is a schematic plan view of the drive magnet configuration ofFIG. 2A;

FIG. 2C is a schematic side view of the drive magnet configuration ofFIG. 2A;

FIG. 2D is a schematic perspective top side view of a bearing magnetconfiguration;

FIG. 2E is a schematic underside view of the bearing magnetconfiguration of FIG. 2D;

FIG. 2F is a schematic side view of the bearing magnet configuration ofFIG. 2D;

FIG. 2G is a schematic underside view of the bearing magnet arrangementof FIG. 2D with an eddy current sensor;

FIG. 2H is a schematic perspective top side view of the bearing anddrive magnet configurations of FIGS. 2A and 2D together with the eddycurrent sensor;

FIG. 2I is a schematic perspective cutaway view of the arrangement ofFIG. 2H;

FIG. 3A is a schematic perspective view of an example impeller from theleft pump side;

FIG. 3B is a schematic cut through view of the impeller of FIG. 3A;

FIG. 3C is a schematic perspective cut through view of the impeller ofFIG. 3A;

FIG. 3D is a schematic perspective view of the impeller of FIG. 3A fromthe right pump side;

FIG. 4A is a graph showing an example of a performance for a heart pumpwith relatively low flow sensitivity;

FIG. 4B is a graph showing an example of a performance for a heart pumpwith relatively high flow sensitivity;

FIG. 4C is a graph showing an example of factors contributing to a pumpcurve;

FIG. 4D is a schematic diagram of an example of blood recirculation at apump outlet;

FIG. 4E is a schematic diagram of an example of the effect of angle ofincidence on blood flow through a pump outlet;

FIG. 5A is a graph illustrating the effect of left pump inlet diameteron the left pump curve;

FIG. 5B is a graph illustrating the effect of right pump inlet diameteron the right pump curve;

FIG. 5C is a schematic cut-away end view of an example of a left pumpvolute outlet throat area;

FIG. 5D is a schematic cut-away plan view of an example of a left pumpvolute outlet throat;

FIG. 5E is a schematic perspective view of an example of a right pumpvolute outlet throat area;

FIG. 5F is a schematic cut-away plan view of an example of a right pumpvolute outlet throat;

FIG. 5G is a graph showing an example of changes in left pump curve fordifferent outlet throat areas;

FIG. 5H is a graph showing an example of changes in right pump curve fordifferent outlet throat areas;

FIG. 5I is a graph showing an example of changes in left pump curve fordifferent outlet throat aspect ratios;

FIG. 5J is a graph showing an example of changes in right pump curve fordifferent outlet throat aspect ratios;

FIG. 5K is a graph showing an example of changes in left pump curve fordifferent outlet throat areas for a constant throat height;

FIG. 5L is a graph showing an example of changes in right pump curve fordifferent outlet throat areas for a constant throat height;

FIG. 5M is a graph showing an example of changes in left pump curve fordifferent outlet throat areas for a constant throat width;

FIG. 5N is a graph showing an example of changes in right pump curve fordifferent outlet throat areas for a constant throat width;

FIG. 5O is a graph showing an example of changes in left pump curve fordifferent outlet throat radial position;

FIG. 5P is a graph showing an example of changes in left pump efficiencyfor different outlet throat areas for a constant throat aspect ratio;

FIG. 6A is a schematic perspective view of an example of the left sideimpeller showing a vane height;

FIG. 6B is a schematic plan view of the left side impeller of FIG. 6Ashowing vane dimensions;

FIG. 6C is a schematic perspective view of an example of the right sideimpeller showing a vane height;

FIG. 6D is a schematic plan view of the left side impeller of FIG. 6Cshowing vane dimensions;

FIG. 6E is a graph showing an example of left pump curves for differentimpeller eye diameters;

FIG. 6F is a graph showing an example of right pump curves for differentimpeller eye diameters;

FIG. 6G is a graph showing an example of left pump curves for differentimpeller left vane heights;

FIG. 6H is a graph showing an example of left pump curves for differentimpeller right vane heights;

FIG. 6I is a graph showing an example of left pump curves for differentimpeller left vane heights with constant inflow area;

FIG. 6J is a graph showing an example of left pump curves for differentimpeller left vane inlet angles;

FIG. 6K is a graph showing an example of left pump curves for differentimpeller left vane outlet angles;

FIG. 6L is a graph showing an example of left pump efficiency fordifferent impeller left vane outlet angles;

FIG. 6M is a graph showing an example of left pump axial pressuresensitivity curves for different impeller left vane outlet angles;

FIG. 6N is a graph showing an example of right pump curves for differentimpeller right vane inlet/outlet angles;

FIG. 6O is a schematic diagram showing flow recirculation behind animpeller;

FIG. 6P is a graph showing an example of left pump curves for differentnumbers of impeller vanes;

FIG. 6Q is a graph showing an example of right pump curves for differentnumbers of impeller vanes;

FIG. 6R is a graph showing an example of left pump curves for differentnumbers of impeller vanes and different vane thicknesses;

FIG. 6S is a graph showing an example of left pump axial pressuresensitivity for different numbers of impeller vanes;

FIG. 6T is a graph showing an example of left pump curves for differentaxial impeller positions;

FIG. 6U is a graph showing an example of right pump curves for differentnumbers of primary and secondary impeller vanes;

FIG. 6V is a graph showing an example of left pump curves for differentprimary and secondary impeller vane configurations;

FIG. 6W is a graph showing an example of right pump curves for differentprimary and secondary impeller vane configurations;

FIG. 6X is a graph showing an example of left pump curves for differentprimary and secondary impeller vane configurations;

FIG. 6Y is a graph showing an example of right pump curves for differentprimary and secondary impeller vane configurations;

FIG. 6Z is a graph showing an example of left pump curves for differentimpeller vane thicknesses;

FIG. 6ZA is a graph showing an example of left pump axial sensitivityfor different impeller vane configurations;

FIG. 6ZB is a graph showing an example of left pump curve for examplethrust bearing configurations;

FIG. 6ZC is a graph showing an example of left pump pressuredisplacement for different impeller vane configurations;

FIG. 7A is a graph showing example pump curves for a number of differentheart pumps;

FIG. 7B is a graph showing flow sensitivity for the heart pumps of FIG.7A;

FIG. 7C is a graph showing pump curves for the left and right pumps foran example heart pump;

FIG. 7D is a graph showing an example of changes in pump curve atdifferent rotational speeds;

FIG. 7E is a graph of an example of the response of the heart pumps ofFIG. 7A to a standing event;

FIG. 7F is a graph of an example of the response of the heart pumps ofFIG. 7A to exercise;

FIG. 7G is a graph showing examples of variations in aortic pressuresfor different pumps;

FIG. 8A is a graph showing examples of pressure variations based onimpeller axial position for different vane outlet angles;

FIG. 8B is a graph showing examples of pressure variations based onimpeller axial position for different vane thicknesses;

FIG. 8C is a graph showing examples of pressure variations based onimpeller axial position for different numbers of vanes;

FIG. 8D is a graph showing examples of pressure variations based onimpeller axial position for different vane heights for a constantimpeller eye diameter;

FIG. 8E is a graph showing examples of pressure variations based onimpeller axial position for different vane heights for constant inflowand outflow areas;

FIG. 8F is a graph showing examples of changes in the pump curve forleft and right pumps for different axial impeller positions;

FIG. 9A is a graph showing examples of radial forces on the impeller fordifferent radial displacements;

FIG. 9B is a graph showing examples of radial forces relative to themagnetic bearing stator locations;

FIG. 9C is a graph showing examples of radial forces for differentexample pumps;

FIG. 9D is a graph showing examples of radial force components fordifferent example pumps;

FIG. 9E is a graph showing examples of radial force components toillustrate the effect of different volute shapes;

FIG. 9F is a graph showing examples of radial force components fordifferent example pumps;

FIG. 9G is a graph showing further examples of radial force componentsto illustrate the effect of different volute shapes;

FIG. 9H is a graph showing examples of radial forces to illustrate theeffect of different pump rotational speeds;

FIG. 9I is a graph showing examples of radial forces to illustrate theeffect of different throat areas;

FIG. 9J is a graph showing examples of radial force components toillustrate the effect of different throat areas;

FIG. 9K is a graph showing examples of radial forces to illustrate theeffect of different throat aspect ratios;

FIG. 9L is a graph showing first examples of radial forces to illustratethe effect of different cutwater angles;

FIG. 9M is a graph showing first examples of radial force components toillustrate the effect of different cutwater angles;

FIG. 9N is a graph showing second examples of radial forces toillustrate the effect of different cutwater angles;

FIG. 9O is a graph showing second examples of radial force components toillustrate the effect of different cutwater angles;

FIG. 9P is a graph showing examples of radial forces to illustrate theeffect of different base circle diameters;

FIG. 9Q is a graph showing examples of radial force components toillustrate the effect of different base circle diameters;

FIG. 9R is a graph showing examples of radial forces to illustrate theeffect of different outer circle diameters;

FIG. 9S is a graph showing examples of radial force components toillustrate the effect of different outer circle diameters;

FIG. 9T is a graph showing examples of radial forces to illustrate theeffect of different cutwater angles and outer circle diameters;

FIG. 9U is a graph showing examples of radial force components toillustrate the effect of cutwater angles and outer circle diameters;

FIG. 9V is a graph showing examples of radial forces to illustrate theeffect of different cutwater angles and base circle diameters;

FIG. 9W is a graph showing examples of radial force components toillustrate the effect of cutwater angles and base circle diameters;

FIG. 9X is a graph showing examples of performance curves for differentexample pumps;

FIG. 9Y is a graph showing examples of efficiencies for differentexample pumps;

FIG. 10A is a series of graphs showing relationship between systemic andpulmonary vascular resistances for different pump curves;

FIGS. 10B and 10C are graphs illustrating the effect of left/rightdesign pressure;

FIGS. 10D and 10E are graphs illustrating the effect of axial movementon balance range;

FIG. 10F is a graph illustrating examples of right pump curves fordifferent right impeller outer diameter;

FIG. 10G is a graph showing examples of different pump curves fordifferent pump configurations;

FIG. 10H is a graph showing examples of different pump curves fordifferent pump configurations;

FIG. 11A is a schematic plan view of a first specific example impellerconfiguration;

FIG. 11B is a graph illustrating a pump curve for the impellerconfiguration of FIG. 11A;

FIG. 11C is a schematic plan view of a second specific example impellerconfiguration;

FIG. 11D is a graph illustrating a pump curve for the impellerconfiguration of FIG. 11C;

FIG. 11E is a schematic plan view of a third specific example impellerconfiguration;

FIG. 11F is a graph illustrating a pump curve for the impellerconfiguration of FIG. 11E;

FIG. 11G is a schematic plan view of a fourth specific example impellerconfiguration;

FIG. 11H is a graph illustrating a pump curve for the impellerconfiguration of FIG. 11G;

FIG. 11I is a schematic plan view of a fifth specific example impellerconfiguration;

FIG. 11J is a graph illustrating a pump curve for the impellerconfiguration of FIG. 11I;

FIG. 11K is a schematic plan view of a sixth specific example impellerconfiguration;

FIG. 11L is a graph illustrating a pump curve for the impellerconfiguration of FIG. 11K;

FIG. 12A is a schematic perspective view of an example of a single VADheart pump;

FIG. 12B is a schematic cutaway side view of the heart pump of FIG. 12A;

FIG. 12C is a schematic cutaway plan view of the heart pump of FIG. 12A;

FIG. 12D is a schematic perspective view of the impeller of the heartpump of FIG. 12A;

FIG. 12E is a schematic perspective view of the magnetic bearing of theheart pump of FIG. 12A;

FIG. 12F is a schematic cutaway perspective view of the impeller of FIG.12A;

FIG. 13A is a graph illustrating changes in drive speed during animplantation process;

FIG. 13B is a graph illustrating changes in vascular resistance duringan implantation process;

FIG. 13C is a graph illustrating changes in blood pressures during animplantation process; and,

FIG. 13D is a graph illustrating changes in pump flows during animplantation process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An example of a heart pump will now be described with reference to FIGS.1A to 1D, FIGS. 2A to 2I and FIGS. 3A to 3D.

In this example the heart pump is a biventricular device which canoperate either as a ventricular assist device to assist function of leftand right ventricles of a subject's heart, or alternatively as a totalartificial heart. It will be appreciated however that whilst referenceis made to a biventricular device this is not essential, andalternatively the principles described herein could equally be appliedto single ventricular assist devices or any other form of blood pump.

In this example, the heart pump 100 includes a housing 110 defining acavity 115. The housing can be of any suitable form but typicallyincludes a main body 110.1, left and right end caps 110.2, 110.3 whichconnect to the main body 110.1, as well as an end plate 110.4 positionedbetween the main body 110.1 and left end cap 110.2. The housing can bemade of any suitable biocompatible material, and can be made oftitanium, a polymer or the like.

The housing 110 includes two inlets 111, 113, for connection to the leftatrium/pulmonary vein and right atrium/vena cava, or left and rightventricles, and two outlets 112, 114 for connection to the aorta andpulmonary artery, respectively. Whilst two inlets and outlets are shown,it will be appreciated that this is in the context of a biventriculardevice, and that a single inlet and outlet can be used for a singleventricular device.

The heart pump 100 includes an impeller 120 provided within the cavity115. The impeller 120 includes a rotor 121 having vanes mounted thereonfor urging fluid from the inlet to the outlet upon rotation of theimpeller 120. In this example, as the heart pump 100 is a biventriculardevice, the impeller includes two sets of vanes 122, 123 each of whichis used for urging fluid from a respective inlet 111, 113 to arespective outlet 112, 114. In this example, the rotor 121 is positionedwithin the cavity 115 to effectively divide the cavity into first andsecond cavity portions, each having a respective inlet and outlet,thereby allowing each to function as a respective pump.

Thus, in the current example, the vanes 122 are used to urge fluid fromthe inlet 111 to the outlet 112, with this being provided on theleft-hand side of the pump in the orientation shown in FIG. 1B, andoperating to provide left-ventricular function, whilst the vanes 123urge fluid from the inlet 113 to the outlet 114 and act to provideright-ventricular function. In this context the first and second cavityportions are generally referred to as left and right cavities, and inconjunction with the impeller 120 provide left and right pumps,respectively. It will be appreciated that in this regard, the terms leftand right refer to the intended ventricular function of the cavities asopposed to the particular orientation of the pump in the Figures, whichis used for illustrative purposes only.

As shown in FIGS. 3A to 3D, the vanes 122, 123 have different profiles,which provide different flow characteristics for the left and right handpumps, as will be described in more detail below. In particular, in thisexample, the left hand vanes 122 are flared outwardly, thickeningtowards an outer circumferential edge of the rotor 121, as well as beingswept so as to be angled away from a direction of rotation of theimpeller, as shown by the arrow R. However, this is not always the case,and for example, for an LVAD the left hand vanes are radially straightand thin, similar to the right sided vanes described below. In contrast,the right hand vanes 123 are generally straight and of a constantthickness, extending radially towards, but without meeting, an edge ofthe rotor perpendicularly. The impact on these arrangements and theparticular dimensions of the vanes 122, 123 and rotor 121 will bedescribed in more detail below.

The heart pump 100 further includes a drive 130 that rotates theimpeller 120 within the cavity 115. The drive 130 can be of anyappropriate form but typically includes a number of coils 131, eachwound on a respective stator 132, supported by a mounting 133, allowingthe drive 130 to be coupled to the housing 110. The drive cooperateswith magnetic material 134 mounted in the rotor 121, with the magneticmaterial being in the form of a number of circumferentially spacedpermanent drive magnets arranged proximate an outer circumferential edgeof the rotor 121 and proximate a face of the rotor 121 facing the drivecoils 131.

An example drive magnet configuration is shown in more detail in FIGS.2A to 2C. In this example, the coils 131 and stators 132 are wedgeshaped and circumferentially spaced around the mounting 133, so as toprovide twelve electromagnets radially aligned with the drive magnets134 in the rotor 121. The drive magnets 134 are arcuate shaped rareearth magnets, circumferentially spaced proximate an outercircumferential edge of the rotor 121, and mounted on a soft iron rotordrive yoke 135.

The heart pump 100 can further include a magnetic bearing 140 includingat least one bearing coil 141 which cooperates with bearing magneticmaterial mounted in the rotor 121 to thereby control an axial positionof the impeller 120 within the cavity 115. In one particular example,shown in more detail in FIGS. 2D to 2F, the magnetic bearing includesthree bearing coils 141, each of which is mounted on a first leg 142.1of respective U-shaped stators 142, with a second leg 142.2 beingpositioned radially inwardly of the first leg 142.1. The stators 142 aremounted to a support 143 and circumferentially spaced 120° apart aroundthe housing so that the first and second legs 142.1, 142.2 align withrespective bearing magnetic material, such as bearing magnets 144, 145,allowing an axial position of the impeller 120 to be controlled.

The bearing magnetic material typically includes first and secondannular magnetic bearing members mounted within and proximate a face ofthe rotor, the first magnetic bearing member being provided radiallyoutwardly of the second magnetic bearing member. In one particularexample, the first bearing magnet material 144 includes an annular softiron material that can be integrally formed with the annular yoke, or anannular permanent magnet 144 mounted on the yoke, and mounted in therotor, proximate an outer circumferential edge of the rotor 121. Thesecond bearing magnetic material is an annular permanent bearing magnet145 mounted radially inwardly of the first bearing member 144, so thatthe first and second bearing members 144, 145 align with respective legs142.1, 142.2 of the stators 142. It will be appreciated the annularmembers could include a plurality of individual elements, such asindividual circumferentially spaced magnets or ferromagnetic elements.Additionally, the use of a magnetic bearing may not be required and canbe replaced by a static physical bearing, or the like.

The drive 130 and magnetic bearing 140 are mounted at opposing ends ofthe housing 110 so that the drive and bearing 130, 140 are providedproximate opposing surfaces of the rotor 121 as shown for example inFIG. 1D, and FIGS. 2H and 2I. In the current example the drive 130 ismounted adjacent the left pump, whilst the bearing 140 is mountedadjacent the right pump, although the opposite configuration iscontemplated. The depicted arrangement has a number of benefits.

Firstly, the inherent attractive magnetic forces between the drive androtor and the bearing and rotor can be configured to substantiallybalance when the rotor is provided at a balance point at a normaloperating speed, which may for example by approximately at a center ofthe cavity under conditions of normal flow.

For example, this arrangement can be configured so that the magneticforces inherent between the drive 130 and impeller 120, and between themagnetic bearing 140 and impeller 120 are matched at an impeller balanceposition within the cavity, which corresponds to a desired position ofthe impeller under conditions of normal flow. This minimises the bearingcurrent required to maintain the position of the impeller 120 within thecavity, hence reducing the amount power required to operate, and inparticular drive and axially position the impeller.

Additionally, as well as having the magnetic forces balance, the forcesgenerated by the drive and bearing can also be configured to provide adesired degree of axial and radial stiffness. In this regard, thestiffness is a measure of the deflection of the impeller 120 from abalance position in response to an external force. In one example, it isdesirable to maximise the radial stiffness so as to maintain theimpeller radially centralised within the cavity and to stop the impellertouching the inner circumferential wall of the cavity. Conversely, asthe axial position of the impeller 120 can be used for flow control, andin particular to allow for passive and/or active response to changes inhemodynamic parameters, a low degree of axial stiffness is preferred.Accordingly, the passive magnetic forces can be configured to assist inmeeting these requirements, as will be described in more detail below.

A further benefit of the above described arrangement, in the context ofBiVAD applications, is that it allows the greater size of the magneticbearing to be accommodated by the smaller size of the right pump cavity.In particular, this allows a gap between a bearing stator and bearingmagnets to be minimized, as no vanes are located in this gap (as opposedto the left side where vanes are located in the magnetic airgap betweenthe drive and the rotor), as will be described in more detail below.However, it will be appreciated that this limits an outer diameter ofthe right pump and thus achievable pressure generation at a givenrotational speed, although for right pumps this is generally not anissue given their lower flow requirements than the left pump. Theapparatus further includes a controller 150 which, in use, is coupled toa sensor 160 and the drive and bearing coils 131, 141. The sensor 160senses an axial position of the impeller 120 within the cavity 115 andcan be of appropriate form.

In the example of FIGS. 2G to 2I eddy current sensors 160 are usedincluding three sensors, each having a coil mounted in a housing 163,circumferentially spaced and aligned with the inner leg 142.2 of themagnetic bearing stators 142. The coil is aligned with a rotorshell/target mounted radially inwardly of a first bearing magnet 144, soas to generate a field therein, with variations in the field beingdetected to determine the separation of the sensor 160 and theshell/target, and hence the rotor 121. However, it will be appreciatedthat other suitable sensors can be used, such as reluctance sensors orthe like, in which case the first permanent magnet 144 might be replacedwith ferromagnetic material, depending on the sensor/bearingrequirements.

In use the controller 150 is adapted to monitor signals from theposition sensor 160 and then control the current supplied to the drivecoils 131 to control rotation of the impeller and to the bearing coils141 to control the axial position of the impeller 120. Thus, theimpeller 120 is acted upon by the fluid pressures in the housing 110,which create a net hydraulic force on the impeller 120. Forces acting onthe impeller 120 are compensated for by the magnetic bearing, with thecontroller 150 operating to control the amount of current supplied tothe electromagnets in the bearing to thereby maintain the position ofthe impeller 120. As such, the current used by the magnetic bearingsystem has a direct correlation to the forces and pressures acting onthe impeller 120. In this manner, changes to the inlet and outletpressures can be detected through the magnetic bearing signals inreal-time.

The controller 150 can also be adapted to provide speed controlfunctionality, altering the rotational speed of the impeller, forexample depending on factors such as fluid pressures within the pump.

The controller 150 can be of any suitable form but typically includes anelectronic processing device 151, an optional memory 152, and aninterface 154 for connecting to the heart pump, each of which areinterconnected by a bus 155, or other similar arrangement. Theelectronic processing device can be any form of electronic processingdevice capable of interpreting signals and causing the drive and bearingto be controlled, such as a microprocessor, microchip processor, logicgate configuration, firmware optionally associated with implementinglogic such as an FPGA (Field Programmable Gate Array), or any otherelectronic device, system or arrangement.

The controller can also implement separate control functionality, forexample separate modules, to thereby control the bearing and drive.

An optional external interface 153 may be provided allowing forinteraction with the controller 150. In the event that the controller ispositioned outside the body this could include an I/O device 153 such asa touch screen or the like, whereas if positioned inside the body thiswould typically be in the form of a wireless communications moduleallowing communication with an external control device.

The above described heart pump is an example of a heart pump that canoperate as a bi-ventricular assist device, providing ventricularassistance to the left and right ventricles, or could act as a totalartificial heart, allowing functionality of the left and rightventricles to be replaced completely. Similar arrangements can also beprovided corresponding to the left or right side pumps only, to therebyprovide left or right ventricular assist devices.

A number of features of a specific example will now be described. Inthis regard, as previously mentioned, the particular design of the pumpcan have an impact on performance characteristics of the heart pump, soselection of appropriate design features can be used to provide desiredflow characteristics.

Examples of desirable flow characteristics include:

-   -   Improved outflow pressure sensitivity (OPS) so that the        magnitude of flow rates are dependent and sensitive to changes        in inflow-outflow pressure differential, so that greater        variations in outflow during alterations in patient        physiological state (exercise, posture change) can be obtained        at a constant impeller rotational speed;    -   Improved axial pressure sensitivity (APS) so that, particularly        for the left pump, flow rates and pressure differentials are        dependent on impeller axial position so that the flow is        adjusted as the impeller moves axially within the cavity, which        can automatically compensate for and balance pressure variations        within the subject's circulatory system;    -   Reduced radial hydraulic forces below 1.2N over the entire a        flow range of 5-8 LPM and more typically 3-12 LPM or even 3-15        LPM, to allowing the radial position of the impeller to be        passively maintained closer to the axis of rotation;    -   Emboli tolerant pump using large impeller flow paths and        clearance gaps to mitigate the potential growth or lodgement of        thrombus in the pump. In particular, to allow emboli that may        originate in the systemic venous system to pass through the        right impeller to then be filtered by the pulmonary network;        and,    -   Improved passive left/right flow balancing through the use of an        appropriate left/right design pressure ratio, high outflow        pressure sensitivity (particularly right sided) and axial        pressure sensitivity (due to impeller movement) to alter the        left/right design pressure ratio in real-time at a given        rotational speed.

In one example, above characteristics are achieved at least in partusing a heart pump with a housing forming a cavity 115 having at leastone inlet 111, 113 aligned with an axis of the cavity and at least oneoutlet 112, 114 provided in a circumferential outer wall of the cavity115. The heart pump further includes an impeller 120 provided within thecavity, the impeller including vanes 122, 123 for urging fluid from theinlet 111, 113 radially outwardly to the outlet 112, 114, a drive 130for rotating the impeller 120 in the cavity 115.

The pump is further arranged to define a flow path through the pump thathas a minimal cross-sectional area of at least 50 mm². In this regard,the term cross-sectional area will be understood to refer to an areadefined perpendicularly to the vector of the fluid velocity of the bloodflow through the pump.

The cross-sectional area is typically achieved through appropriateconfiguration of the inlet, outlet and impeller, such as the impellervane configuration. Providing a flow path of at least 50 mm² provides anumber of advantages.

For example, this sizing more closely mimics the cross-sectional area ofthe human circulatory system, making the heart pump more effective froma biocompatibility perspective. In this regard, the human circulatorysystem, and in particular the heart valves, for an adult typically has aminimum cross-sectional area of 300 mm², and handles instantaneous flowrates of up to 25 Liters per min (LPM), leading to a flow velocity of upto 1.4 m/s. As the heart pump typically operates over a range of 2 LPMto 8 LPM, and up to 12 LPM during exercise, this leads to a typical flowvelocity through the pump of 0.6 m/s to 4 m/s. While this is high at anupper end of the range, this is still significantly improved overtraditional heart pumps, which can have a flow path of less than 20 mm²,leading to a velocity of up to 10 m/s. Thus, this represents asignificant improvement over traditional arrangements.

Furthermore, in practical scenarios, the cross-sectional area can beincreased to 100 mm², with patients typically not exercising and hencerequiring less than 8 LPM, leading to an upper limit on the blood flowvelocity of 1.3 m/s, typical of normal human blood flow.

Additionally, the increased cross-sectional area of the flow pathinherently leads to an emboli tolerant pump, mitigating the potentialgrowth or lodgement of thrombus in the pump. Additionally, the largecross-sectional areas lead to improved outflow pressure sensitivity(OPS) due to reduced flow induced resistive losses, and reduced radialhydraulic forces. This can also assist with improved passive left/rightflow balancing in BiVAD/TAH applications, although it will beappreciated that other advantages are also applicable in LVADapplications.

In one example, the heart pump further includes a magnetic bearing 140for controlling an axial position of the impeller 120. However, it willbe appreciated that benefits can also be obtained in non-magneticallylevitated impeller configurations, such as hydraulically suspendedimpellers, or shaft mounted impellers.

Whilst the cross-sectional area of the flow path is at least 50 mm², itcan be at least 75 mm²; at least 100 mm², at least 125 mm², at least 140mm², at least 150 mm², at least 200 mm², and up to 300 mm².

In one example, the heart pump is configured to provide a pump having apump curve having a gradient of less than −20% over a defined flowrange, such that a change in pressure of 10 mmHg across the pump causesa change in flow rate of at least 2 litres per minute (LPM), the definedflow range being between 5 LPM and 8 LPM and more typically between 3LPM and 12 LPM or 15 LPM.

Such a gradient can be achieved for a pump that generates a pressurehead that is, for a left ventricular pump, between 60 mmHg and 100 mmHgat 6 LPM, between 70 mmHg and 90 mmHg at 6 LPM and more typicallyapproximately 80 mmHg at 6 LPM, whereas for a right ventricular pump,this can be between 10 mmHg and 30 mmHg at 6 LPM, between 15 mmHg and 25mmHg at 6 LPM and more typically approximately 20 mmHg at 6 LPM.

Thus, a heart pump is provided that has a pump curve with a much lowergradient than traditional pumps, as shown for example in FIG. 4B, versusthe conventional heart pump gradient shown in FIG. 4A. As a result, theheart pump demonstrates high flow sensitivity, so that even minorpressure variations caused by changes in physiological state (outflowresistances) of the subject result in a high change in flow rate throughthe pump, which in turn provides a correction mechanism to ensure thatflow rates are suitable from a physiological perspective.

For example, if pressures at the left pump inlet 111 increase (due to areduced upstream pulmonary resistance), this results in a higher flowthrough the left pump, thereby returning the pressure at the pump inlet111 to a normal level. Thus, a highly sensitive left and/or right pumpwill maximize outflow at a constant rotational speed during increasedpatient activity (exercise). Additionally, in the case of abiventricular device, this can also help maintain left/right flowbalance over a wider and more extreme range of physiological imbalances(posture change/Valsalva), by allowing the flow rate through the leftand right pumps to adapt solely on the basis of pressures.

Thus, this allows a more pressure sensitive heart pump to be provided,which can maintain balanced flow and provides more cardiac output over awider range of patient daily activities at a constant rotational pumpspeed.

In one example, when the heart pump provides at least partial leftventricular function, the heart pump can have a pump curve having agradient less than at least one of −25%, −30%, −35% and more typically−40%, but can be up to −200% or even −500%. In the case of a right pump,the pump curve can have a gradient less than at least one of −30%, −35%,−40%, −75%, −100% and more typically 150%, but can be up to −200% oreven −500%. Thus, in one example biventricular device, the left andright pumps can have different pump curves, which can further assist inflow balancing. However, this is not intended to be limiting and a rangeof different pump curve characteristics can be used. For example, whilstthe above gradient values are particularly suited for a total artificialheart scenario, if the pump is being used as a single VAD, such as anLVAD or RVAD, a gradient can be up to −200% or even up to −500%.

In this regard, it should be noted that the pump curve refers to themeasured pressure from the inlet to the impeller to the outlet of theimpeller. In practice, the shape (straight or bent/curved) and diameterof the inlet and outlet pipe/cannula will also affect this pump curve,but this is not accounted for in the explanation above. Thus the abovegradient values are in respect of fluid flow from the inlet to theoutlet of the pump, and are independent of connection of the pump to thesubject's body.

The sensitivity to changes in outflow due to pressure variations on theheart pump is related to the pump curve of the pump, which incorporatesvarious losses (friction, hydraulic, leakage/recirculation) which are inturn dependent on design parameters of the impeller/volute, includingthe characteristic resistance, throat area, volute angle, vane angles,vane numbers, vane dimensions, or the like.

It should be noted that, as will be described in more detail below,these design parameters also have an impact on other aspects of pumpoperation, and in particular, parameters suitable for ensuring a highaxial flow sensitivity can also have a number of other beneficialeffects.

For example, this can help ensure that the heart pump provides greatersensitivity to pressure generated due to alterations in axial positionof the impeller 120. Additionally, parameters that contribute to animproved pump curve can result in low radial hydraulic forces over therange of intended outflow, which enables suspension support in a radialdirection by passive reluctance restoration forces generated by an axialmagnetic bearing, without requiring additional stabilisation, such asthe use of a hydrodynamic journal bearing, which can have adversephysiological and hemocompatible consequences.

Consequently, this configuration allows for large gaps to surround thecircumference of the impeller (typically 2-4 mm) which results in lowshear stress and thus improved biocompatibility. Additionally, whencombined with large axial gaps (250 to 350 μm and typically 300 μm), asignificant amount of washout flow between the cavities can occur. Inthe TAH application, this results in a shunt of oxygenated blood fromthe high pressure left cavity to the low pressure right cavity. Thedegree of shunt flow is not considered clinically significant, howevermeasures can be used to attenuate the magnitude of flow, such as theinclusion of fins or vanes or spiral groove bearings to oppose shuntflow. For the LVAD application, a large hole located in the centre ofthe impeller will direct this leakage flow back into the main flow path.In both cases, this leakage flow is effective in providing washout ofthese clearance gaps, thus reducing the potential for stagnant flow andconsequent thrombus formation.

In one specific example, the ability to shunt flow from the left to theright pump can have significant benefits during installation of thepump, as will be described in more detail below.

Parameters that contribute to an improved pump curve also typically leadto large passages through the impeller vanes (at a range of 120 mm²-300mm² and typically with a minimum of 150 mm² on the left and at a rangeof 500 mm²-1500 mm² and typically with a minimum of 650 mm² on theright) used to reduce the characteristic resistance of the flow path andalso allows for the passing of emboli through the pump. This is ofparticular importance for any emboli originating in the systemic venoussystem to pass unimpeded through the right side of the device, to thenbe filtered by the arterioles in the pulmonary network of the lungs.Furthermore, these large areas greater than 120 mm² may reduce theincidence of vonWillebrand factor degradation, due to lower fluidvelocity and thus shear stress at maximal flow rates.

Specific examples of configurations will now be described.

Improved Outflow Pressure Sensitivity (OPS)

The pump curve for a heart pump is dependent on a wide range ofdifferent factors, as shown for example in FIG. 4C. In particular, eachpump has theoretical performance characteristics defined by Euler'sequation, which are derived from angular momentum considerations basedon the design of the impeller vanes, as indicated by the Euler-line.Deviation from this theoretical performance results from factors suchas:

-   -   leakage resulting from flow between impeller vanes and the walls        of the impeller cavity;    -   friction between the bloodflow and walls of the pump;    -   recirculation between the cutwater and the impeller as shown in        FIG. 4D;    -   recirculation within the outlet volute/throat area leading to        impeded flow as shown in FIG. 4E;    -   inlet port diameter and thus fluid recirculation, particularly        at flows below the best efficiency point (BEP);    -   volute throat magnitude (area) and aspect ratio (Width×Height)    -   impeller eye diameter (ID1);    -   the use of secondary splitter vanes;    -   impeller vane number and fluid entrance/exit area; and,    -   angle of flow incidence entering the impeller vanes and        interacting with the volute cutwater tongue.

Accordingly, by controlling parameters of the physical construction thatcan influence these factors, this can be used to configure a desiredpump curve, and in turn a high degree of outflow pressure sensitivity.In one particular example, a desired pump curve is obtained by reducingfriction losses by increasing the throat area, altering the vane outletangle and increasing recirculation losses at low flows while reducingrecirculation losses at high flows.

Each of these will now be considered individually, although it will beappreciated that there is some interaction between the individual pumpparameters and each factor, which leads to constraints on the selectionof each parameter. Accordingly, in practice the pump parameters must beconsidered collectively, and reference to each factor individually isfor the purpose of illustration only.

Friction Losses

Friction losses increase with flow rate due to viscous boundary layerproduction and increased effective blockage throughout the flow path.Reducing the ratio of boundary layer thicknesses to the main flow patharea created by flow areas interacting with the relatively viscous fluid(blood) within the device reduces characteristic resistance/blockages toflow, especially at high flows. This may be achieved by increasing thecross-sectional area of flow paths through the heart pump.

Increasing the cross-sectional area of flow paths through the heart pumpcan be achieved by increasing the cross-sectional area/diameter D1 ofthe inlets 111, 113. In this regard, as inlet diameter increases,shutoff pressure decreases due to additional recirculation losses at theinlet. However at high flows, friction losses are reduced withincreasing inlet diameter, leading to a flatter pump curve. It should benoted that the inlet diameter that affects recirculation is the diameterof the port as it enters the pump cavity, and hence as blood flows ontothe impeller. In other words this is the inlet diameter downstream ofthe connection to the cannula or cuff that attaches to the body and anybend in the inlet, which can be of any diameter. Examples of this areshown in FIGS. 5A and 5B, which show example left and right hand pumpcurves for a range of different inlet diameters ID. For example, for theright hand pump, increasing the inlet diameter ID by 2 mm drops theshutoff pressure by ˜1.3 mmHg and reduces the gradient by 0.1 mmHg/LPM.

This effect has limited influence above 20 mm, so in one example, theinlet has a diameter of at least one of at least 10 mm, at least 15 mm,less than 30 mm, less than 25 mm, between 18 mm and 22 mm and moretypically approximately 18 mm to 20 mm. This contrasts to traditionalarrangements in which smaller diameters, such as 9-10 mm or lower, areused. It will also be appreciated that different inlet diameters may beused for the left and right pumps respectively.

Additionally, throat area A_(th) of the throats 112.1, 114.1 of the leftand right pump outlets 112, 114, shown in FIGS. 5C and 5D and 5E and 5F,respectively can be increased in order to maximise pressure at higheroutflows from the pump at a given impeller rotational speed. In a rotaryblood pump, the throat area is traditionally less than 60 mm², whichcorresponds to creating a best efficiency (design) point (BEP) at 5-6LPM. A number of different throat areas were trialed, and a graphshowing examples of different pump curves for different throat areaswith the same impeller configuration is shown in FIGS. 5G and 5H for theleft and right pumps respectively. For these examples the aspect ratio(width vs height) of the throat is kept constant.

In this case, for the left pump, shutoff pressure increases as throatarea Ath and width (W) decreases, leading to a slightly steeper curve.In this instance, it is apparent that flow above 9 LPM starts to chokeif the throat area is below 120 mm², but performs similarly at greaterareas, with limited benefit being seen at areas above 140 mm².

In contrast, for the right pump, reducing the throat area A_(th) causesthe flow to choke below 200 mm², with significantly reduced performanceat 150 mm² due in part to the smaller vane height. Limited benefit isseen above 200 mm², suggesting that an optimum lies around this valuefor outlet throat area for the right hand pump.

These results highlight that a larger throat area has a flatter pumpcurve. As the throat area becomes larger, greater fluid deceleration atthis region results at flows below BEP<1, as well as more of arecirculation volume and so more static pressure energy is convertedinto kinetic energy. Whilst a smaller throat area reduces this effect,it creates more fluid acceleration (which reduces static pressure) andthus turbulence and blockage at higher flows above BEP. These effectswere observed up until 140 mm² on the left and 200 mm² on the right,above which only marginal alterations in the performance curve wereobserved. Accordingly, whilst a larger throat area provides a flattercurve, this needs to be balanced by efficiency and radial hydraulicforce generation considerations as described later.

Accordingly, in one example, for a left pump, the outlet 112 has an areaof at least one of at least 60 mm², at least 80 mm², at least 120 mm²,between 60 mm² and 250 mm², between 120 mm² and 160 mm², between 140 mm²and 160 mm², between 140 mm² and 250 mm², between 130 mm² and 150 mm²,and approximately 140 mm² or 150 mm². In the case of a right pump, theoutlet 114 has an area of at least one of at least 100 mm², at least 130mm², between 130 mm² and 250 mm², between 130 mm² and 230 mm², between170 mm² and 210 mm², between 150 mm² and 200 mm², between 170 mm² and210 mm2, between 175 mm² and 200 mm², approximately 233 mm², 200 mm² or150 mm². The choice of a larger right pump throat area than left assistsin producing a more OPS right pump than left pump. That is to say, thethroat area Ath of the right pump should be larger than that of theleft, in general.

In addition to an absolute throat area, an aspect ratio of the outletand in particular the ratio of width to height, also has an impact onflow through the outlet. In this regard, a higher throat area with alarger height has a greater length of cutwater tongue, which can in turnlead to separation eddies due to incidence mismatch having a greaterimpact on flow blockage.

Examples of the impact of change in throat aspect ratio, for a fixedthroat area, are shown in FIGS. 5I and 5J for the left and right pumprespectively.

As can be observed in Figure SI, maintaining the same throat area butaltering the aspect ratio (W:H) alters the shape of the performancecurve, with a larger aspect ratio generally producing flatter pumpcurves in the case of the left pump, which has a spiral volute. A largeraspect ratio (AR) is characterised as locating the outer wall of thethroat area at a larger outer wall diameter (OD3). Since the location ofthe cutwater is unchanged (54 mm) at the base circle diameter (BCD), thevolute spiral angle is thus increased to make the transition around theimpeller circumference, and was between 1.7° and 3.4° for this aspectratio range. A larger volute angle is generally more favourable to matchthe fluid off-flow discharge angle created by the flow exiting theimpeller at high flows (higher meridional radial flow velocity at thesame impeller rotational tangential velocity) thus reducing the level ofincidence mismatch at high flows and thus maintaining pressure. Thisfluid off-flow discharge angle is approximately 4.5°-11.5° for flowsbetween 3-12 LPM@2100 RPM for an impeller with a 20-90° outflow vaneangle (β₂).

The reduced throat area height also reduces the cutwater wall height,and thus volume of separation downstream of the cutwater tongue at thesehigher flows, and thus reduced influence of throat blockage by thiscutwater/flow velocity incidence mismatch. At part (significantlyreduced) flows, the volute angle and thus flow incidence has a largermismatch, which causes more recirculation volume upstream the cutwatertongue (between the impeller and the cutwater) as well as at the outerwall of the throat, leading to retarded pressure generation. Reducingthe throat area width (and consequently the volute angle) by reducingthe diameter of the outer wall OD3 maintains pressure at part flowshence steepening the curve and exhibiting a larger shutoff pressure. Thereduction in volute angle in this case matches the off-flow angle atpart flows better, and thus reduces the separation and thusrecirculation volume at both the cutwater and volute outer wall at partflows. It was found that increasing the AR>1.4-1.6 had minimal effect oncreating a flatter pump performance curve.

In the case of the right volute of the right pump, it is aconcentric/circular volute, hence the volute angle is always 0°. Asshown in FIG. 5J, as the throat area width increases, shutoff pressuredecreases and the curve is flatter. This observation breaks down as theaspect ratio AR>0.65, whereby the height (and corresponding vane height)is reduced below 17.5 mm which reduces the ability for the impeller tocreate pressure over all flow rates. Therefore widths lower than 11.40mm start to choke flow >11 LPM, whilst at widths higher than 11.40 mm(and corresponding height reduced) head pressure suffers. Hence, anaspect ratio AR of ˜0.65 is ideal for a 200 mm² throat area A_(th).

A number of different throat areas and geometries were trialed, andresults are shown in FIGS. 5K to 5N, with the throat area geometry beingexpressed as a width and height. In these cases, the throat area Ath wasvaried and either width or height was kept constant.

For the left pump, as shown in FIGS. 5K and 5M, increasing the voluteouter diameter and hence the throat area width (and thus volute angle)whilst maintaining throat height retards the ability to generatepressure at part flows due to the greater degree of separation ofrecirculating fluid at both the cutwater tongue and outer wall of thethroat. Resistance to flow is reduced at high flows, leading to aflatter curve as width and throat area A_(th) is increased. A marginaldifference was observed for a throat area A_(th) above 140 mm² for theleft pump.

For the right pump, shown in FIGS. 5L and 5N, a similar observation wasmade, whereby performance did not alter much between all throat areasA_(th) when height was maintained at 17.5 mm until below 150 mm². Henceallowing for a smaller throat area A_(th) of 150 mm²-175 mm² for theright pump, results in a 8.6 mm-10 mm throat width. This in turn assistsin passing the right pump outlet between the magnetic bearingcomponents, which can in turn have a larger size.

In contrast, increasing the throat area A_(th) by maintaining the width(and thus volute angle) and increasing the height had a more pronouncedeffect on pressure generation and pump curve gradient.

For the left pump, identical shut off pressures were generated at eachthroat area A_(th), however increasing the throat area A_(th) leads toan increase in generated pressure at high flows due to reducedresistance and thus results in a flatter pump curve. For the rightimpeller, shutoff pressure was found to decrease as throat area A_(th)increased by increasing height and maintaining width, whilst morepressure was generated at high flows, due to a reduction in resistance.Detrimental performance was observed when increasing the throat areaA_(th) above 200 mm² and corresponding to a height above 17.5 mm.

Finally, for a constant throat area of 140 mm², a number of differentleft pump throat area locations were trialed, with the throat arealocation being expressed in relation to an inner (base circlediameter—BCD) and outer throat area wall diameter (OD3), compared to thepump cavity as a whole.

As shown in FIG. 5O, each arrangement has a generally similar gradient,particularly at high flows. However as already observed, as the throatwidth W reduced, shutoff pressure increased leading to a steepergradient. Maintining the throat width (and thus volute angle) byincreasing both the base circle diameter BCD and outer wall diameter OD3resulted in similar pressure generation at high flow, but retardedpressure generation at part flows. Since a larger base circle diameterBCD increases recirculation area between the cutwater and the impeller,flow does not need to decelerate as much at part flows, hence lesskinetic energy is converted to pressure. Since the width W is the same,the amount of recirculation adjacent the throat outer wall is similar,so the reduction in pressure is due to the cutwater recirculation.

Increasing the base circle diameter from 54 mm to 58 mm whilstmaintaining the outer wall diameter at 71 mm (thus reducing both widthand volute angle but increasing height) reduces the generation ofpressure at part flows due an increase in the recirculation areadownstream of the cutwater.

The larger base circle diameter BCD allows for fluid to recirculate pastthe cutwater with a lower velocity at part flows (but still highervelocity than that in the throat area) and thus less acceleration, henceless pressure losses (meaning lower radial force at part flows, asdescribed later) and thus more pressure generated at shut off. Howeverthis means the pump curve is slightly steeper.

Increasing the outer wall diameter OD3 from 67 mm to 71 mm whilstmaintaining the BCD slightly improves pressure generation at high flows.Additionally, increasing the outer wall diameter OD3 helps to generatethis pressure at high flows due to the larger volute angle created andhence closer match to the impeller off flow angle at high flows.

The width was therefore determined as a major contibuting factor foraltering the pump curve gradient, with larger widths returning flattercurves. Coupling this with larger base circle diameter BCD furtherreduced pressure generation at part flows and thus flatten the curvefurther.

Taking the above into account, for a left hand pump, the outlet 112typically has a substantially rectangular cross-sectional shape and awidth to height aspect ratio of at least one of between 1:2 and 2:1,between 1:1 and 2:1, between 1:1 and 1.8:1, between 1.1:1 and 1.6:1 andapproximately 1.4:1. The base circle diameter at the throat is typicallyat least one of at least 50 mm, less than 100 mm, less than 80 mm,between 50 mm and 74 mm, between 54 mm and 64 mm, and approximately 60mm. The outer wall diameter at the throat is typically at least one ofat least 40 mm, less than 100 mm, less than 80 mm, between 50 mm and 80mm, between 65 mm and 76 mm, and approximately 71 mm. It will beappreciated that these values are in respect of a 50 mm rotor, and thatsmaller diameters would be required for smaller diameter rotors. Thus,in general the base circle diameter at the throat is between 108% and125% of the rotor diameter and the outer wall diameter is between 130%and 160% of the rotor diameter.

In one preferred example, for the left pump the throat aspect ratio(width:height) is at least 1.4:1, the throat area is at least 150 mm²,the base circle diameter is not greater than 120% of the rotor diameterand the outer wall diameter is at least 130% of the rotor diameter.

In one example, for a right hand pump, the outlet 114 typically has asubstantially rectangular cross-sectional shape and a width to heightaspect ratio of at least one of between 1:3 and 1:1 and approximately0.45-0.65:1. The cavity typically has a diameter of a least one of atleast 20 mm, at least 25 mm, less than 40 mm, less than 30 mm, between27 mm and 29 mm and, approximately 28 mm. These values are in respect ofa 24 mm-25 mm diameter rotor, so that the right cavity diameter isapproximately 112%-116% of the cavity diameter.

In one preferred example, for the right pump, the throat aspect ratio(width:height) is between 0.45:1 and 0.65:1, the throat height is about17.5 mm, the throat area is at least 150 mm², and more typically about175 mm², and the base circle diameter is about 28 mm. The larger theBCD, the more space for rotor radial displacement before potentialtouchdown, however the least space for magnetic bearing components,hence an optimum of 28 mm was reached for this embodiment.

Choosing the best efficiency point BEP to be at the higher end of theexpected operating range (9-12 LPM), as shown in FIG. 5P, by selecting alarger throat area maintains a flatter curve for the intended operatingrange of 3-12 LPM, which is counter to traditional arrangements in whichthe BEP is typically set at mean blood flows for the subject of about5-6 LPM, corresponding to a resting condition, thereby minimising theaverage energy usage over the entire operating flow range.

It should be noted that whilst the configurations that produce theflatter curves may cause recirculation eddies at low flows, these can becounteracted with regular impeller speed modulations to increase flow toat least BEP flow each cycle to completely wash out any recirculationeddies.

Additionally, the cross-sectional flow area through the pump isinfluenced by the impeller inner eye diameter ID1 of the impeller, shownfor the left and right hand pumps in FIGS. 6B and 6D respectively, andvane height, shown as b₂ and b₁ in FIGS. 6A and 6C, for the left andright pumps, respectively. In general, a larger inner diameter for agiven vane height results in a larger cross-sectional area flow path.Similarly, a larger vane height for a given impeller inner eye diameterID1 results in a larger cross-sectional area flow path. Increasing theflow path area generally reduces resistance and produces a greaterefficiency for the same rotational speed.

An example of the impact of impeller inner eye diameter ID1 will now bedescribed with reference to FIGS. 6E and 6F.

Increasing the inner eye diameter for a given vane height results in alarger flow path area and reduced resistance to flow (especially highflow) but at the expense of a reduction in Euler pressure generation ata given RPM (Stepanoff, A. J. 1957). That is, a larger diameterincreases vane circumferential velocity u₁ at the inner diameter and theflow area is greater and hence meridonial inlet velocity c_(m1) islower, thus reducing the theoretical head over all flows. However thishas a greater influence at lower flows, whereby the fluid inlet velocityc₁ is larger since the meridonial inlet velocity c_(m1) is smaller, butimpeller circumferential velocity to is the same (and vane inlet angleβ₁ is fixed). Hence the shutoff pressure is lower and pump curve isflatter at low flows. Furthermore, the inlet vane incidence mismatch isgreater at larger diameters due to this larger circumferential velocity,leading to a rapid reduction in pressure generation as soon as forwardflow begins to develop. For the purpose of these experiments, the inletvane angle β₁ was kept constant at 64°, thus at these larger diameters,the mismatch of incidence angle effectively replicates a larger inletangle (at a smaller diameter) and as such greater mismatch especially atlow flows. As flow rate increases, the incidence mismatch decreases,hence pressure is generated and the pump curve becomes flatter. Pressuregeneration is also maintained at high flows with increasing ID1 due tothe larger inlet flow areas and hence lower flow resistance.

In this regard, an impeller eye diameter ID1 of 20 mm, for a vane heightof 2 mm gives a fluid inlet area through the vanes of 125.6 mm². Incontrast, for a 25 mm-30 mm impeller eye diameter ID1, the area becomes157 mm²-188.4 mm² respectively.

Increasing the impeller inner diameter ID1 is most effective at bothflattening the pump curve and also reducing the pressure generation at agiven rotational speed. This characteristic is observed whenever theimpeller inlet diameter ID1 exceeds the pump inlet diameter D1, and canbe utilised to great effect when attempting to create the preferredleft/right design pressure ratio. Furthermore, the resultant largerimpeller inlet area provides an easier passage for systemic venousemboli to pass, and also reduces flow velocities and thus shear stressesin this region.

Accordingly, a preferred impeller eye diameter ID1 for the left pump isin the region of 25-30 mm (for an inlet port diameter of 20 mm), but maybe up to 40 mm and as little as 10 mm.

For the right pump, increasing the impeller eye diameter ID1 results ina reduction of shutoff pressure, whilst reducing below 14 mm chokesflows above 6 LPM, resulting in a steep curve at high flows (flow at theouter circumference enters the top of the spinning vanes).

Accordingly, for the right pump the impeller eye diameter ID1 istypically selected to be 16-18 mm (for an inlet port diameter of 18 mm),but can be as low as 10 mm and as high as 20 mm.

An example of the impact of vane height will now be described withreference to FIGS. 6G and 6H, which show pump curves for different vaneheights for the left and right pump respectively.

Increasing the impeller vane height (whilst maintaining impeller inletdiameter) results in a larger flow path area and reduced resistance toflow (especially high flow), without the consequent reduction in Eulerpressure generation. In fact, the reduction in meridonial inlet velocityc_(m1) caused by increasing the vane height assists in developingpressure by reducing the ‘u₁×c_(u1)’ term of the Euler equation. Hence,as can be seen in FIGS. 6H and 6I, increasing vane height also raisespressure over all flow rates. The reduced radial flow velocity caused byan increased height also exacerbates the inlet vane angle mismatch,particularly at low flows, thus also contributing to a flatter pumpcurve.

Major changes to the gradient were observed in the left impeller whenheight is reduced below 2.0 mm, thus this vane heights are in a suitablerange for the left side TAH application due to its pressure generationbeing in the range required to produce the designed left/right designpressure ratio. A 4.0 mm vane height suits the VAD application sinceonly minor changes in gradient were observed with larger heights. Theselarger heights require a special motor design to maintain high efficientoperation at relatively large magnetic airgaps. A vane height of 15-17.5mm is suitable for the right side TAH application. Since minorperformance improvements were observed with larger heights, a 15 mmheight allows venous emboli to pass the top of the vanes.

Additionally, as shown in FIG. 6I, by increasing the impeller eyediameter ID1 and reducing the vane height to maintain an inlet area of125 mm² and outlet area of 194 mm², the effect of flow choking forheights smaller than 2.0 mm is mitigated, hence all PQ curve gradientsare similar at high flows. As already observed, a larger impeller eyediameter reduces the overall Euler pressure, especially at low flows,leading to a flatter pump curve.

In general terms, pressure at high flows can be raised by increasing thevane height and thus reducing resistance to flow, and pressure at lowflows can be reduced by increasing the impeller inner diameter to valueslarger than the inlet port diameter, thus leading to a flatter pumpcurve. However, it should also be noted that an increased vane heightreduces the axial pressure sensitivity (especially when the impeller eyediameter ID1 is maintained), as will be described in more detail below.

As axial pressure sensitivity is more important for the left hand pumpin the TAH application, the resulting vane height is significantlyreduced. Consequently, for the left pump for the TAH application, thevane height is at least 1.0 mm, but less than 5 mm, between 1.3 mm and 3mm, between 1.5 mm and 2.5 mm, between 1.7 mm and 2.3 mm andapproximately 2.0 mm In contrast for the right pump has a vane height ofat least one of at least 10 mm, less than 30 mm, between 15 mm and 20mm, between 17 mm and 18 mm and approximately 17.5 mm.

Impeller Inlet/Outlet Vane Angle/Volute Cutwater Angle

Losses due to flow vector and impeller vane/volute cutwater incidenceangle mismatch is minimal at the BEP, which results in minimalrecirculation and eddies in the vicinity of the inlet vanes andcutwater, as shown in FIG. 4D, which increase above and below BEP.Blockage due to incidence separation at the impeller inlet occurs atflows either side of the flow rate and impeller speed for which they aredesigned. Blockage and separation at the cutwater tongue increases aboveBEP as fluid separates downstream of the cutwater, hence contributing toa reduced effective throat area, greater fluid acceleration and thusreduced pressure generation. A larger volute cutwater angle willincrease the flow rate for which downstream separation will occur at thecutwater, whilst at the same time, will increase the separation upstreamof the cutwater at lower flows below the design point. Both of theseeffects lead to a flatter pump curve as seen above (larger aspect ratioand thus larger volute angle leads to a flatter curve). Vane outletangle affects also pressure generation as flow rate increases, and thuspump curve gradient, since V_(m2) increases for a larger flow and thusc_(u2)′ reduces, leading to a lower Euler pressure. This characteristicis even more pronounced as outlet angle is reduced, meaning that thepump curve gradient steepens as outlet angle reduces.

Selection of the impeller vane inlet angle alters the gradient of thepump curve due to separation and blockage mentioned above. Furthermore,the higher the vane inlet angle, the larger the c_(u1)′, and thus thelower the Euler pressure that is generated (Stepanoff, A J., 1957),particularly at low flows since the incidence on the inlet angle is mostdeviated from the design flow for a larger angle. However the larger theangle, the reduced resistance to flow through the pump, which dominates.

An example of the impact of vane inlet angle on the left pump is shownin FIG. 6J. From this it is apparent that increasing the vane inletangle from 64° to 84° creates marginally more pressure over all flows,and a flatter curve, due to reduced resistance, especially at higherflows above 4 LPM. A smaller inlet angle appears to choke flow after 4LPM due to excessive flow path blockage due to a mismatched incidenceangle which creates a steeper pump curve.

To utilize the effect of inlet angle on shaping the pump curve to beflatter, the left pump vanes have an inlet angle β₁ of at least one ofless than 90°, greater than 60°, between 80° and 90°, between 82° and86° and approximately 84°.

For reasons explained above, a larger outlet vane angle leads to aflatter pump curve gradient. An example of this effect of left vaneoutlet angle β₂ is shown in FIG. 6K.

For a set vane height of 2.0 mm and thus for impeller inlet and outletflow passages of 125 mm² and 194 mm² respectively, reducing vane outletangle significantly disrupts pressure generation and thus steepens thePQ curve gradient. The difference between 80° (−2 mmHg/LPM) and 60°(−2.5 mmHg/LPM) in terms of gradient between 5-10 LPM is however,minimal. Efficiency is also comparable as shown in FIG. 6L. Performanceand efficiency of the 20° vane impeller is worse than the otherconfigurations, however this result is indicative of an axial gap abovethe vane tips of 500 μm. When this gap is reduced to 100 μm, the shutoffpressure and efficiency is comparable, albeit at a steep gradient. Thisdrastic alteration in performance returns an amplified APS.

The axial pressure sensitivity is shown in FIG. 6M, which shows that thegenerated pressure is reduced by 3 mmHG for 60° vanes@6 LPM, so 80°outlet angle should be selected for flattest curve. The 20° vanes createthe highest axial pressure sensitivity over the 500 μm range, which canbe explained by the additional leakage transiting over the impellervanes causing an opposition to the main forward flow.

Thus, this demonstrates that increasing vane angle β₂ flattens the pumpcurve and increases efficiency at a 500 μm gap but decreases axialpressure sensitivity. Accordingly, for the left hand pump, where someaxial pressure responsiveness is desirable, the impeller has a vaneoutlet angle of at least one of less than 60°, greater than 20°, between30° and 50°, between 35° and 45°, between 38° and 42° and approximately40°.

An example of the impact of vane inlet angle on the right pump is shownin FIG. 6N. The lower the vane inlet/outlet angle, the higher theshutoff pressure and lower the maximum flow pressure. The arrangementwith a 60° inlet angle and 72° vane outlet angle creates 0.5 mmHg morepressure@6 LPM and otherwise performs very similar to an impeller with90° inlet and outlet angles and thus an acceptable vane inlet anglewould be between 60°-90° for the right pump.

Thus, in contrast, for the right hand side impeller, a greater pressuresensitivity and thus flatter pump curve is desired whilst axial pressuresensitivity is less required, hence vane angle, and in particular bothinlet and outlet vane angle are maximised. Accordingly, the right sideimpeller has a vane inlet angle β_(j) of at least one of greater than75°, less than 115°, between 80° and 100° and approximately 90° and avane outlet angle β₂ of at least one of greater than 75°, less than115°, between 80° and 100°; and approximately 90°.

It will also be appreciated from the above that the cutwater angle canheavily influence flow within the pump and hence the shape of the pumpcurve. In one example, for the right pump, the cutwater angle CW₂ shownin FIG. 5F is at least one of between 90° and 180°; between 90° and135°; between 0° and 90°; between 45° and 90°; between 45° and 135°;between 60° and 80°; and approximately 70°. In one example, for the leftpump, the cutwater angle CW₁ shown in FIG. 5D is at least one of between0° and 70°; between 30° and 50°; between 40° and 45°; between 35° and45°; between 45° and 50°; between 0° and 60°; and approximately 45°.

Impeller Recirculation/Slip

Impeller recirculation/slip arises due to recirculating flow between thehigh pressure leading face of the impeller vane and the low pressuretraining edge of the next vane, creating a relative eddy, as shown inFIG. 6O, resulting in a decrease in pumping efficiency and hence flow.

Vane Number

Impeller recirculation/slip depends on the physical barrier provided bythe vanes. The number of vanes can lead to an alteration of impellerrecirculation/slip magnitude. Examples of this are shown for the leftand right pumps in FIGS. 6P and 6Q, respectively.

For the left pump, when the vane width is maintained and more vanes areadded, the effect on P/Q and efficiency of adding more vanes than eightis not significant. However, pressure (at 500 μm) suffers when less thaneight vanes are used, and is significantly disrupted when less than sixvanes are used. Although more vanes provide more pressure generation,this is offset by the increased flow resistance caused by blockage,especially at high flows.

For the right pump six vanes and eight vanes perform similarly, whilstfour vanes have poor performance Increasing the number of vanes to tenincreases pressure generated at 6 LPM by ˜1 mmHg but max flow pressureis the same as six and eight (thus the gradient is steeper)

Thus the larger the vane number, the greater the pressure generation,leading to a flatter pump curve when impeller outlet area is maintained(by altering the outlet vane thickness z₂). However when the area is notmaintained, as shown in FIG. 6R, the pressure generating benefits ofadditional vanes are offset by the increased resistance caused by areduction in impeller exit area, especially as flow increases.

In the example of FIG. 6R, maintaining the exit flow path area at 194mm² (by altering the vane exit width z) whilst altering the vane numberhad significant impact. However the larger vane number reduces theindividual surface area of each vane and thus may compromise theperformance of any thrust bearing. Efficiency was comparable foreight-twelve vanes. The most number of vanes (twelve) generated thehighest pressure overall with the flattest gradient (although gradientwas similar with eight-twelve vanes from 5-12 PLM). Accordingly, in oneexample, eight vanes are used to drop pressure by ˜6 mmHg for the samespeed and allow sufficient surface area for a backup thrust bearing.

These relative changes in left impeller performance were observed with a500 μm axial gap between the casing and the tip of the vanes. When theaxial gap is reduced to 100 μm, the lower vane number impeller had asimilar shutoff pressure to the larger vane number impeller (albeit at asteeper pressure gradient as flow increased due to the impeller slip).The relationship between pressure and axial gap is shown in FIGS. 6S and6T. As such, as described in detail later, the axial pressuresensitivity of a lower number of vanes (four) is higher and is preferredfor the TAH application, since performance can be altered by varying theaxial gap and gradient flattened by selecting a larger outlet angle.

Impeller recirculation/slip depends on the physical barrier provided bythe vanes, and hence the tall vanes 123 on the right hand side of theimpeller significantly reduce impeller recirculation/slip.

Whilst a larger number of vanes can lead to a further decrease inimpeller recirculation/slip, this can reduce the cross-sectional flowpath available to blood traversing the pump, and so accordinglysecondary vanes can be provided, the secondary vanes having an increasedinner diameter, so that the flow path cross-sectional area is notreduced and thus resistance to flow not increased, whilst the effectivebarrier provided by the impellers is increased.

An example of this for the right pump is shown in FIG. 6U. In thisexample, half of the primary vanes that start at a diameter (16 mm) arereplaced with secondary vanes that begin at a larger diameter (19 mm).This interrupts pressure generation at flow <8 LPM, which leads to aflatter pump curve between 5-8 LPM. Flow above 12 LPM is choked less andpressure at higher flows is slightly higher. Increasing the number ofvanes from eight to ten increases pressure generation by lmmHg@6 LPM butwith a slightly steeper gradient. The flattest curve is produced by afour primary four secondary vane configuration. This gradient iscomparable to an 18 mm impeller eye diameter ID1 arrangement, but witha >1 mmHg pressure generation, which helps to achieve the desiredleft/right design pressure ratio at a greater overall pump efficiency.

However, the reduced resistance path provided by the alteration ofprimary to secondary vanes (with a larger inner diameter) is offset bythe reduced effective vane area imparting energy on the fluid (vaneloading/length). Thus whilst the addition of the splitter vanes resultsin a slightly flatter curve at high flows, this comes at the expense ofa reduced pressure generation over all flows.

The effect of the inner diameter of the secondary impeller vanes willnow be described. As shown in the example of FIG. 6V, which shows pumpcurves for the left pump, adding secondary vanes reduces the pressuregenerated over the whole flow range due to a reduced Euler pressuregeneration by the split vanes at a larger diameter, and in particularreduces the pressure by 12 mmHg@6 LPM. This is a similar observationfrom increasing the impeller eye diameter ID1 of all vanes, and relatesto the reduced vane loading. Thus, using splitter vanes can beconsidered as a means to reduce the pressure generation of the leftimpeller into the range targeted for a suitable TAH left/right designpressure ratio, whilst maximising OPS.

Adding a set secondary vanes having an inner eye diameter ID2 of 30 mmflattens the pump curve gradient especially above 8 LPM due to reducedblockage. Using an eye diameter ID2 greater than 30 mm adversely affectspressure generation and gradient. Accordingly, in one example, thepreferred approach is to use a four primary four secondary vaneconfiguration with a secondary vane eye diameter of up to 30 mm.

In the case of the right pump, as shown in FIG. 6W, adding secondaryvanes with an eye diameter of 19 mm reduces pressure at low flowsleading to a marginally flatter pump curve, but pressure generated at 6LPM is comparable. A secondary vane with an inner eye diameter of 22 mmreduces pressure over all flows (approaching performance of four vanesonly) but does not raise the gradient and as such is not preferred.

The vane configuration (number, inner diameter etc) and outlet angle β₂also have an impact on flow as will now be described with reference toFIGS. 6X and 6Y.

In the case of the left pump, eight full vanes at 60° outlet anglehaving an impeller eye diameter of 20 mm, created 10 mmHg more pressure@ 6 LPM than four primary and four secondary vanes. The effect of thefour primary and four secondary vanes is to reduce pressure generationdue to the increased diameter of the split vanes and thus reduced Eulerhead generation by these vanes.

It was observed that six full vanes at 80° vane outlet angle β₂ create apump curve of similar pressure and gradient to eight vanes at 60° vaneoutlet angle β₂, however the latter starts to choke flow >8 LPM andhence the six vane gradient is flatter in this region. Therefore it isgenerally preferred to reduce the vane number and increase the vaneoutlet angle β₂ rather than utilizing splitter vanes for the left pump.It can be understood that splitter vanes may be utilized to reduce thedesign pressure generation at a given rotational speed, whilst notcompromising gradient, which occurs when vane number alone is reduced.

The flattest pump curve and thus greatest OPS for the left impeller ofthe TAH application is obtained for the eight vanes having 80° vaneoutlet angle β₂ with an impeller eye diameter of 30 mm, thus, in oneexample, an impeller eye diameter ID1 of 30 mm and splitter ID2 of 35 mmis used in conjunction with an 84° vane inlet angle β₁ and 80° vaneoutlet angle β₂ to reduce pressure generation to a range suitable forthe targeted left/right pressure ratio whilst creating the flattest pumpcurve.

In the case of the right pump, reducing the inner diameter of theprimary vanes to 12 mm increases shutoff pressure, but steepens thecurve from 8 LPM as flow is choked entering the impeller eye. Adding anextra primary and secondary vane, to use a five primary and five secondvane configuration raises pressure by 1 mm over all flows and maintainsgradient, whilst removing a primary and secondary vane to use a fourprimary and four second vane configuration reduces pressure by 1 mm overall flows but maintains gradient. Finally, removing one primary vane andadding two to use a three primary and six second vane configurationreduces pressure by 0.5 mm over all flows but maintains gradient. Ingeneral, it is preferred to reduce the number of vanes to increase thefluid passage between the vanes to allow emboli to pass unimpeded. Inone example, a four primary and four second vane configuration is usedwith impeller eye diameter ID1 of 16 mm and 19 mm respectively.

For the left hand pump, the impeller typically includes a number ofprimary vanes having an inner diameter of at least one of at least 10mm, less than 40 mm, between 15 mm and 35 mm, between 25 mm and 35 mmand, approximately 25-30 mm, a number of secondary vanes, the secondaryvanes having an inner diameter of at least one of at least 20 mm, lessthan 40 mm, between 30 mm and 40 mm, and, approximately 35 mm and anouter vane diameter of at least one of at least 20 mm, less than 60 mm,between 45 mm and 55 mm, between 48 mm and 52 mm, and approximately 50mm. The impeller can includes an equal number of primary and secondaryvanes, at least three primary and secondary vanes, less than six primaryand secondary vanes, and more typically four primary and four secondaryvanes.

For the right hand pump, the impeller includes a number of primaryvanes, the primary vanes having an inner diameter of at least one of atleast 10 mm, less than 25 mm, between 10 mm and 20 mm, between 14 mm and18 mm and, approximately 16 mm, a number of secondary vanes, thesecondary vanes having an inner diameter of at least one of at least 10mm, less than 25 mm, between 15 mm and 25 mm, between 18 mm and 20 mm,and approximately 19 mm and an outer vane diameter of at least one of atleast 15 mm, less than 40 mm, between 20 mm and 30 mm, between 22 mm and27 mm, and approximately 24 mm or approximately 25 mm. The impellertypically includes an equal number of primary and secondary vanes,between three and five primary vanes, and more typically four primaryvanes, between three and six secondary vanes and more typically foursecondary vanes. The lower the number of vanes, whilst still optimisingperformance, allows for a larger fluid passage and thus more area forany possible deep vein thrombus emboli to pass unobstructed through theright impeller.

In addition, a further reduction in impeller recirculation/slip can beachieved by increasing the vane thickness, particularly towards theouter circumferential edge of the impeller. This is not required for theright hand side, due to the vane height. However, this is beneficial forthe left hand side, so that the vanes thicken towards the outercircumferential edge of the impeller, resulting in an outer edgethickness of at least 5 mm, less than 20 mm, between 6 mm and 14 mm,between 7 mm and 8 mm and approximately 7.5 mm for an eight vaneimpeller. In contrast, for the right hand pump, the vanes have an outer(and substantially constant) thickness of at least one of at least 0.5mm, less than 3.0 mm, between 0.75 mm and 2 mm, and 1.5 mm Too thin andfatigue cracking may occur at the base of the long struts. For thisreason, a suitable fillet connection from the strut to the rotor baseshould be utilised.

An example of the impact of impeller vane thickness for the left pumpwill now be described with reference to FIGS. 6Z and 6ZA.

As shown in FIG. 6Z, a change in vane width has a significant effect onefficiency and pressure gradient. For example a 1 mm vane outer widthresults in a pump curve gradient of −1.5 mmHg/LPM while 4 mm and 7.5 mmwidths result in a −2 mmHg/LPM gradient and >7.5 mm width has a gradientof −3.5 to −4 mmHg/LPM. Based on this 7.5 mm is acceptable to allow alower pressure generation (<20 mmHg) at 2100 RPM.

Turning now to FIG. 6ZA, this shows that the change in vane width has aneffect on axial pressure sensitivity. The improvement for a vane widthof greater than 10.9 mm is marginal, suggesting that the vane thicknessshould be at least 10.9 mm for an impeller with eight vanes if APS is tobe maximised.

Thus, increasing vane thickness at the outer circumference has the addedbenefit of increasing axial pressure sensitivity. However an additionalbenefit of a thicker vane is for the inclusion of a backup hydrodynamicthrust bearing configured to act between the left impeller vanes and theflat left casing face between the rotor and the motor. Such ahydrodynamic bearing would not be in functional operation until theimpeller approached the left casing wall. This may occur during anexcessive shock force, or in the unlikely event that the magneticbearing suspension ceases operation. In such cases, the motor wouldcause the impeller to continue to rotate, and the hydrodynamic bearingwould provide non-contact suspension, until such time as the magneticbearing can be restored.

Whilst the 7.5 mm vane thickness discussed is sufficient, a thicker vanewould result in a larger hydrodynamic bearing surface area, but at theexpense of more resistance to flow and thus a steeper pump curve. Tomitigate this issue, the secondary vane thickness can be reduced (to 1.0mm), which allows the primary vane to increase to 14 mm beforesignificantly impacting flow resistance. Further, the primary and secondvane impeller eye diameter ID1, ID2, of 25 mm, 40 mm respectively, canbe made to start at a larger diameter to take advantage of this featureand reduce the pressure generated, particularly at low flows, to matchthe targeted left/right design pressure ratio and return a flatter pumpcurve.

Example thrust bearing configurations will now be described withreference to FIG. 6ZB and FIG. 6ZC.

These examples highlight thickening the four primary vanes from 7.5 mmto 14 mm and reducing the four secondary vanes from 7.5 mm to 1.0 mmresults in a 3 mmHg greater generation of pressure at shutoff. Thegradient from 5-10 LPM reduced from 2 mmHg/LPM to −2.5 mmHg/LPM.Increasing the primary vanes further to 18 mm started to choke flow offthe impeller and thus steepened the gradient to −3 mmHg/LPM. It istherefore aacceptable to thicken primary vanes to 14 mm and reducesecondary to 1.0 mm to trade −0.5 mmHg/LPM gradient for a larger andmore efficient thrust pad.

Whilst the addition of the splitter vanes helps to improve OPS, it comesat the expense of a reduced APS. To improve this optimisation of OPS andAPS, the splitter vanes can be removed and the four primary vanesthickened to 15 mm or 18 mm. The inner eye diameter ID1 is increased to30 mm which both reduces pressure generation (particularly at low flows)and improves OPS. Outlet vane angle is increased to 80° and inletincreased to 84° to further restore the OPS. Vane height could also beincreased to 2.5 mm.

Results of the above described configurations are shown in FIGS. 7A to7G.

In particular, a heart pump created in accordance with the abovedescribed arrangements, denoted as BVCR in FIG. 7A, provides goodoutflow pressure sensitivity up to and above 3.5 LPM/mmHg over thedefined operation range of 5-12 LPM, whereas traditional rotary bloodpumps BP1, BP2, BP3, BP4 maintain approximately 0.1 LPM/mmHg over thisrange. In particular, the gradients are BP1=80−46/3=−11.33 mmHg/LPM,BP2=−10.6 mmHg/LPM, BP3=−17.3 mmHg/LPM and BP4=−13.3 mmHg/LPM, whereasin the described configuration, the pressure gradient is slightlypositive from 5-8 LPM.

As also shown in FIG. 7C, the above described configuration furtherresults in a significantly different pump curve between the left andright pumps, for example due to different vane configurations, which aspreviously indicated can assist in providing differential flow controlbased solely on pressure variations.

FIG. 7D highlights how the pump curve gradient is consistent atdifferent rotational speeds, helping to maximize the degree of controlthat can be maintained. Under these conditions, the gradient between5-10 LPM is around −0.25-0.33 mmHg/LPM. A peak hydraulic efficiency of57% occurs at 12 LPM, with an efficiency at 7.5 LPM of 55%, meaningthere is little decrease in efficiency below BEP, so that overall energyusage is not majorly impacted.

The beneficial effect of the greater outflow pressure sensitivity ishighlighted with respect to a change in posture (standing event), inwhich a subject goes from an at rest sitting condition, to standing. Asshown in FIG. 7E, an initial reduction of arterial pressure by just 15%was observed for BVCR compared to up to 40% for other rotary blood pumpdesigns BP1, BP2. Thus, the above described arrangement can help preventexcessive drops in blood pressure caused by the onset of activity,helping prevent lightheadedness, dizziness or the like.

Further, FIG. 7F shows how an increase of outflow by 35% was observedin-vivo during a transition to exercise of a cow with a heart pump on atreadmill, compared to 2-8% for other rotary blood pump designs. Thiswas achieved without additional control of the heart pumping, meaningthe pump is automatically compensating for physiological changes withoutrequiring additional control of rotational speed.

A further beneficial effect is observable in the pressures shown in FIG.7G. In a single VAD application SVCR, the transmitted pulsatility of thenative heart is better preserved. In an MCL experiment, the naturalpulse pressure was preserved to 35 mmHg in comparison to just 5-10 mmHgin previous rotary blood pump designs. When an exercise/stand-upcondition was simulated (by a reduction in SVR), MAP dropped by just 15mmHg as opposed to 25-30 mmHg in the previous designs, which provides agreater tolerance to orthostatic hypotension. Importantly, diastolicpressure is lowered allowing arterial vessels vital relaxation time,which may reduce the incidence of hemorrhagic stroke, vascularmalformations and gastrointestinal bleeding.

Accordingly, it is apparent that the use of a high outflow sensitivityto inflow pressure can significantly improve the ability of the heartpump to accommodate natural pressure changes within a subject, and thatthis can be achieved through the selection of appropriate parameters,including increasing flow path cross-sectional area to reduce friction,selection of an optimum vane angles, and configuring the impeller toreduce impeller recirculation/leakage.

Improved Axial Pressure Sensitivity (APS)

As previously mentioned, it is useful to maximise axial pressuresensitivity to allow for flow to be adjusted based on the relative axialposition of the impeller within the cavity. This allows for an expandedrange of left/right design pressure ratio at a given rotational speed.In this regard, the axial pressure sensitivity is largely dependent onthe amount of leakage between the impeller vanes and the cavity housing,and so accordingly, the axial pressure sensitivity is largely dependenton the impeller vane parameters and in particular the vane outlet angle,the vane thickness, vane number, and the vane height.

Some leakage is desirable as its alteration contributes to axialpressure sensitivity, in particular by allowing the amount of leakage tobe controlled by adjusting a clearance between the vane and housing.

Vane Outlet Angle

The lower the outlet vane angle is, the more pressure sensitivity isobserved due to the inwardly radial component of flow passing over thetop of the vane, between the vane and the housing, which acts to opposethe main forward flow.

The most influential factor for BTG AP sensitivity was found to be thevane outlet angle β₂, as shown by the graph of FIG. 8A.

A backward swept curved vane of 20° has almost double the pressuresensitivity than straighter radial vanes. This may be explained by thebreakdown of velocity components of the leakage flow over the vanes. Apure radial vane exhibits a circumferential velocity vector alone,whilst a backward swept vane produces a velocity component in the inwardradial direction, opposing forward flow through the impeller.

Vane Thickness

The thickness of the vane contributes to resistance to flow over thevane as the vane approaches the casing, with a thicker vane increasingfriction to flow, and reducing the ability for blood to flow over thevane, hence resulting in enhanced axial pressure sensitivity.

Thus, a change in vane width has an effect on axial pressure sensitivityas shown in FIG. 8B. However, an increase in vane thickness only leadsto a minor increase in sensitivity for a vane thickness over 10.9 mmConsequently, the axial thickness of the outer edge of the vanes can beselected to be 10.9 mm if the vane thickness is to be otherwiseminimised whilst maintaining increased axial sensitivity, but otherwiseany thicker vane can be used.

Vane Number

A similar factor applies to the number of vanes. In this instance, asshown in FIG. 8C, decreasing the number of vanes leads to an increase inaxial pressure, so fewer thicker vanes are more sensitive than a largernumber of thin vanes.

Vane Height

The height of the vane also contributes to axial pressure sensitivity,with a higher vane reducing the ability of blood to flow between thevane and housing and hence reducing axial pressure sensitivity, (whilstincreasing outflow pressure sensitivity and efficiency). Examples ofchanges in sensitivity for different vane heights for constant impellereye diameter ID1, and constant inlet and outlet areas are shown in FIGS.8D and 8E respectively. From this, it is apparent that this sensitivityis insensitive to left vane height when the inlet (125 mm²) and outlet(194 mm²) flow areas of the impeller are maintained by increasing theinner eye diameter ID1 and reducing the outer diameter vane thicknessz₂. However it is very sensitive to height when the inlet and outletareas vary as a factor of the height.

As previously described, the right hand pump impeller vane parametersare defined to flatten the pump curve as much as possible, in particularthrough the use of the large vane height and 90° vane angles. As aresult, axial pressure sensitivity is minimised. In contrast, for theleft hand pump, a degree of gradient is maintained for the pump curvedue to the lower vane height, reduced vane outlet angle, and increasedvane thickness, effectively sacrificing outflow pressure sensitivity infavour of axial pressure sensitivity. An example of this is shown inFIG. 8F, in which incremental movements of the impeller 120 within thecavity result in movement of the pump curve of the left hand pump, butless alteration for the right hand pump.

In practice for the left hand pump, a change in axial position of 200 μmcauses a change in flow rate of at least one of at least 1 LPM, at least2 LPM, less than 4 LPM and between 2 LPM and 3 LPM, whereas for theright hand pump, the change in flow rate is at least one of at least 0.2LPM, at least 0.5 LPM, less than 2 LPM and between 0.5 LPM and 1.5 LPM.

Radial Stabilisation

Radial destabilization forces arise from hydraulic force, rotor weightand shock disturbances. Hydraulic force is generated by the unevenpressure distribution of blood around the impeller periphery caused byacceleration and deceleration of fluid within the volutes leading to theoutlets 112, 114. The direction and magnitude of this force is steady,and dependent on the volute type (circular, spiral, splitter) and variesaccording to operational flow rate. This force becomes unsteady in thecase where the device is operated in pulsatile outflow mode bymodulating the rotational speed, whereby the radial forces fluctuatewith instantaneous outflow. This instantaneous outflow may exceed theaverage flow rate range required for circulatory support in humans (3-12LPM), possibly reaching maximum instantaneous flow rates of 25 LPM. Incontrast, the force generated by rotor weight and shock disturbancesdepends on the mass of the rotor and whilst the weight is steady inmagnitude but varies in direction with patient position, the shock forceand direction depends on the magnitude and direction ofaccelerations/decelerations applied to the device. Furthermore, thelatter is generally considered an impulse disturbance and thus thedamping of the fluid and surrounding bodily tissue helps to mitigate themagnitude of this force experienced by the system.

Accordingly, it is necessary to provide a mechanism for radialstabilisation that can accommodate these radial forces and prevent, orreduce the chance of impact between the impeller 120 and housing 110. Inparticular, to achieve non-contact suspension, the technique chosen forradial suspension must be able to generate sufficient forces tocounteract hydraulic, gravitational and shock disturbances.

One technique to achieve radial suspension capacity involves thereduction of the radial clearance around the periphery of the rotor 121.This creates a hydrodynamic journal bearing, which essentially relies onthe generation of high and low pressure zones around the rotor hub uponthe onset of rotation. The force capacity and stability of the journalbearing is determined by a number of factors such as rotational speed,rotor diameter, rotor mass and fluid viscosity. Thus, radial clearance(often in the range of 50-150 μm) and the length of the journal (8-30mm) are the main parameters remaining to alter to improve journalbearing performance. The addition of the inherently small clearance actsas a restriction to flow from the high pressure left chamber to the lowpressure right.

Since there is only one rotating part and a full suspension is desired(no direct contact between the rotor and the casing), a leakage flowwill form from the left to right cavity, driven by the pressuredifference from the left to the right cavities. Whilst this has anegligible side effect for the patient, it is tended to lead traditionalarrangements to implement a journal bearing and minimise separationbetween the rotor and housing.

However, this creates a region of high shear stresses, which in turn cancase rupturing of red blood cells leading to hemolysis, activatedplatelets, and destruction of vonWillebrand Factor. Accordingly, it isdesirable to provide an alternative mechanism for maintaining theimpeller radial position. Furthermore, the hydraulic force and operatingspeed influence the function of the journal bearing, as such it ischallenging to maintain functionality over the full range of flow rateand speeds required for the TAH application.

Magnetic Bearing

Whilst not essential for a heart pump, magnetic suspension of a bloodpump can significantly assist in improving biocompatibility. Suspendingthe rotor of the device with magnetic forces allows for larger clearancegaps between moving surfaces and thus lower shear stress exposure to theformed elements in the blood. Hence a lower degree of blood damage isexpected.

In the current example pumps, active magnetic suspension is provided inthe axial direction. However radial suspension is also required. Toaccount for the anticipated radial hydraulic forces that are createdfrom the pump configuration, a non-contact hydrodynamic journal bearingmay conventionally be required. However the smaller gaps (50-150 μm)required for this bearing to function could potentially harm the formedelements of the blood.

The active axial magnetic bearing however can provide a (weaker) radialrestoring force than a hydrodynamic bearing. However if the externalradial forces can be reduced sufficiently over the operating flow rangeof the device, and the radial gap increased sufficiently, this solutionsuffices. A much larger gap (in the order of 2-4 mm) can then beutilized. Hence the design of the pump should attempt to minimize thisradial hydraulic force over the operating flow range.

This criteria is not mutually exclusive to the requirement for improvedoutflow pressure sensitivity and improved hydraulic efficiency, andhence should be considered in the final selection of pump components.

Thus, some radial suspension capacity is provided by the axial magneticbearing and motor system. The attractive force generated by thepermanent magnets toward the stators of both motor and bearing alsogenerate a ‘passive’ radial restoring reluctance force with rotor radialdisplacement. Thus, at the same time that the bearing stabilizes therotor axially, by balancing the attraction force from the motor and theresultant hydraulic forces acting on the rotor, it also generates astable restoring radial force when the rotor is eccentric to thedrive/bearing axis.

The radial force generated by the magnets is a function of the angle ofthe force vector resulting from the attraction between the bearing andmotor stators and the rotor magnets. When there is no eccentricitybetween the rotor and the pump casing there is no radial force and theaxial force is maximum. With increasing eccentricity the vectordecomposes in the axial and radial direction, with small change in theforce magnitude. This leads to a roughly linear increase in the radialforce with increasing eccentricity. As eccentricity further increases,the favourable change in vector angle is compensated by a decrease inthe vector magnitude and the slope of the radial force decreases withincreasing eccentricity, as shown in FIG. 9A.

It is therefore apparent that the bearing configuration has the abilityto provide at least some radial stability. Additionally, as the forceincreases with displacement, it is desirable to allow for greater radialmovement of the impeller through the use of a larger radial gap comparedto that provided for in journal bearing arrangements. This in turn canincrease leakage between the left and right cavities. However, this canbe compensated for by reducing the total axial movement of the rotorfrom 1 mm to 0.5-0.7 mm (typically 0.6 mm, but can be as low as 0.1 mm)and thus operating with comparatively lower axial gaps between thestepped face of the rotor and the casing.

In particular, the rotor typically has an outer circumferential wallspaced from an inner cavity wall by at least one of an average distanceof at least 2 mm, an average distance of less than 8 mm, an averagedistance of less than 5 mm and an average distance of approximately 4mm. This separation refers to a minimum distance between the rotor andthe housing, which typically occurs between the rotor and BCD orcutwater. It will be appreciated that at other locations the spacing canbe significantly larger, such as up to 8 mm In general the minimumspacing is dependent on the size and in particular the diameter of theimpeller, and accordingly, the separation is typically at least 5% ofthe impeller rotor diameter, typically less than 40% and more typicallyapproximately 16% of the impeller rotor diameter.

Shock disturbances are damped by the fluid viscosity in the gap, andthis arrangement alone is sufficient to maintain bearing stability aslong as hydraulic forces are minimised.

Whilst the magnitude of the radial force directly contributes to thechance for the rotor to touchdown the casing in the radial direction,the direction of this force over the flow range can also assist whenutilising the passive reluctance forces generated from an axial magneticbearing. In this regard, the magnetic bearing system has an inherentpassive radial restoring stiffness which increases when the force isacting directly towards a bearing stator. An example of the magnitude ofthe restoring force relative to the location of the bearing stators MB1,MB2, MB3 is shown in FIG. 9B.

From this it will be appreciated that it is desirable to manipulate themaximum radial force vectors at high and low flows to urge the rotortowards the bearing stators, thus enabling the system to take advantageof increased radial stiffness. Whilst the MB stator locations could thenattempt to be adjusted to suit, it is more desirable to manipulate theforce vector, which can be achieved through suitable selection of voluteparameters, particularly cutwater angle, shape this force vector, aswill now be described.

Volute Configuration

When considering volute configuration it is important to take intoaccount the physical dimensions of the resulting pump. In this regard,the goal of an implantable heart pump is to minimize device implantationsize to maximize the population that can receive circulatory support.The outer diameter OD3 of the volute casing is usually the largestradial dimension of the device, hence should be minimized withoutcompromising performance.

It is known from turbo machinery theory that different volute collectorprofiles produce distinct pressure distributions around the impeller.More specifically, three types of volute are commonly employed inindustry: circular, single and double or split volute. Whereas thecircle volute is the simplest of all three, it generates an unevenpressure distribution and therefore comparatively higher radial forces,especially at higher flows.

Another volute type is a single/spiral type. The single volute has itscross-section increasing linearly (or as a function to conserve fluidangular momentum) around the circumference, therefore providing anefficient conversion from fluid velocity into pressure and, hence, aneven pressure distribution around the impeller. Because the evenpressure distribution is only occurring at the pump BEP, the use of thesplit volute is an effective way of balancing uneven pressuredistribution, therefore generating lower radial forces for all operationpoints at the cost of larger viscous losses. In traditionalsingle/spiral volute types, the cutwater angle CW is 0°. However, toallow for a sufficiently large throat area A_(th), the outer diameterOD3, shown in FIG. 5F, must increase significantly, meaning the devicemay be too large to fit in smaller patients. This can be counteracted byincreasing the CW angle, allowing OD3 to be reduced, allowing for asmaller overall device and allowing for a larger effective A_(th).

To illustrate the effect of volute configurations, severalconfigurations were systematically tested in a force test rig, whilstkeeping the same rotor geometry, 10.6 mm length, for the sake ofcomparison. Among the configurations were circular volutes and singlevolutes, with parameter variations of base circle diameters (BCD),cutwater angles (CW), volute angles (VA) outer throat area walldiameters (D3) as well as throat areas (A_(th)) made up of variations inheight (H) and width (W) to form different aspect ratios (AR). Allconfigurations tested are listed in TABLE 1 below.

TABLE 1 BCD BASE OD3 Ath CW VA CIRCLE OUTER THROAT W H AR CUTWATERVOLUTE DIAMETER DIAMETER3 AREA WIDTH HEIGHT ASPECT ANGLE ANGLE # (mm)(mm) (mm²) (mm) (mm) RATIO (°) (°) V1 59 59 50 16.4 3.4 4.8 64 0 LV145254 67 159.49 13.99 11.40 1.2 45.6 2.51 LV1453 58 71 143.08 14.6 9.80 1.542 2.31 LV1454 54 67 138.18 14.1 9.80 1.4 42 2.48 LV1455 56 69 140.7314.36 9.80 1.5 42 2.40 LV1527 58 71 154.97 14.62 10.60 1.4 42 2.31LV1528 60 71 154.76 14.6 10.60 1.4 44.2 1.91 LV1532 58 71 169.51 15.4111.00 1.4 44.5 2.33 LV1534 58 71 154.88 12.44 12.45 1.0 36 2.27 LV153554 71 154.76 14.6 10.60 1.4 37 3.20 LV1536 60 71 182.13 14.629 12.45 1.244.28 1.91 LV1538 60 73 157.68 14.875 10.6 1.4 42.15 2.24 LV1540 58 7160.95 5.75 10.6 0.5 0 2.04 LV1541 58 76 87.45 8.25 10.6 0.8 0 2.83LV1542 59 76 100.91 9.52 10.6 0.9 20 2.78 LV1543 60 76 153.02 14.43610.6 1.4 40 2.73 LV1544 62 76 145.86 13.76 10.6 1.3 40 2.32

The results of all measured forces are shown in FIGS. 9C and 9D, withthe pump performance curves and efficiencies being shown in FIGS. 9X and9Y. The outcome of these results is discussed in more detail in thefollowing sections.

Volute Shape

The effects of different volute shape on radial forces are shown inFIGS. 9E and 9F. This highlights that the circular volute (V1) producesunmanageable radial forces over the desired operating range of 3-12 LPM.Using a single volute with a large throat area designed for a BEP at 12LPM flattens the pump curve, and also significantly reduces the forcesat high flows, however raises them at lower flows due to fluiddeceleration. LPM

Further example circular and single volute configurations are shown inFIG. 9G. Whereas some designs were provided with a radial gap of theorder of 0.1 mm to mimic the journal bearing influence, many designs ofconsiderably larger radial gaps were tested for providing an idea aboutthe forces the magnetic bearing system would have to passively overcome.

The first observation about the force curves is that higher forces areexpected with increasing flow. This is valid for the circle volute inthe whole tested range and for the single volutes after their turn overat the best efficiency point, where in both cases, fluid in the throatarea region is required to accelerate. In addition, increasing the basecircle diameter (BC) of either the circle or single volute decreases theforces, especially at higher flows (circular) and low flows (single).The circle volute geometry with the lowest force was the one with nojournal bearing and thus a larger radial gap. In this volute, a 3 mmradial gap was left between rotor and volute wall, which effectivelyincreased the volute cross-section area. This shows that not necessarilythe base circle alone is a major determinant of the radial forces butthe area where the rotor discharges the fluid pumped by the vanes(compare, for instance, circle BC 59 with circle BC59 (no journalbearing)), and the angle that the cutwater starts.

Secondly, single volutes produce, in general, less radial forces thancircle volutes at design flows, with the rule of increasing the volutecross-section still holding. Moreover, increasing volute cross-sectionmeans an increase in throat area, altogether shifting the point oflowest force to higher flows. This result is in accordance withtraditional turbo machinery theory. Furthermore, increasing thecross-section area yielded flatter force curves, effectively loweringthe radial force, especially at high flows. This is of particularinterest for the TAH application pump, since it enables stable operationat more demanding hemodynamic conditions like during exercise (flowsabout 12 LPM).

Accordingly, on left hand pumps, which tend to generate the greatestforce, the casing may include a single volute, with this typically beingconfigured to generate a maximum radial force of less than 1N.

Rotational Speed

As shown in FIG. 9H, alterations in speed had minimal effect on theradial force shape/profile, rather shifting the minimum force point tothe right as speed increased, This characteristic is of particularbenefit when operating the device in a pulsatile outflow mode, wherebyspeed is increased to both increase outflow pressure and flow, resultingin lower forces at higher flow rates than would occur at a lower speed.

Effect of A_(th) on Radial Force

The impact of throat area A_(th) will now be described with reference toFIGS. 9I and 9J.

When comparing LV1527 and LV1532, whereby A_(th) increases whilst AR,CW, BCD and D3 remain the same, a shift in minimum resultant radialforce to higher flows is observed. Considering the individual X and Yforce components, it can be observed that the larger A_(th) translatesto a lower+Y force, due to reduced acceleration in the throat and thusless reduction in pressure that would counteract unbalanced forcesacting in this direction. Likewise, at low flows, the larger area at thethroat produces larger fluid deceleration in this region andcorrespondingly raises the local pressure in this region, contributionto a larger force in the—Y direction Minimal changes to X forces areobserved with this configuration.

Despite the larger A_(th) in LV1543 than LV1542 and LV1541 (153 mm²compared to 101 mm² and 87.5 mm²), increasing the BC from 58 mm to 59 mmto 60 mm and CW from 0° to 20° to 40°, increasing the AR from 0.8 to 0.9to 1.4 (while maintaining H and OD3), significantly reduced forces atlow flows and increased them slightly at flows above 12 LPM, resultingin the minimum force point reducing from 11 LPM to 8 LPM, and forcesremaining below 0.8N from 3-12 LPM. The deceleration of fluid at thecutwater at low flows due to a small A_(th) and thus volume upstream ofthe cutwater dominates the production of part flow forces, particularlyin the X direction opposing the cutwater.

Accordingly, this aligns with the preferred throat areas discussedpreviously.

Effect of Throat Geometry (AR) on Radial Force (H and W)

The impact of throat geometry, and in particular aspect ratio AR willnow be described with reference to FIG. 9K.

When comparing LV1527 and LV1534, whereby AR and CW are both reduced,while A_(th), BCD and OD3 are maintained. the minimum resultant forcepoint is shifted to higher flows and increases in magnitude. Inaddition, resultant forces at low flows are increased, while forces athigh flows are somewhat reduced.

A similar trend is observed with LV1528 and LV1536, and LV1543 andLV1527 whereby A_(th) is increased and AR decreased by simply increasingthe height (H) and maintaining CW, BCD/OD3 and throat width (W).

The throat geometry or aspect ratio in turn can set the rotor height.Examination of these results show that the ideal rotor height isapproximately 10 mm, with smaller rotors resulting in a boundary layerblocking outflow at the throat and thus increase the radial force athigh flows due to excessive fluid acceleration, whereas rotors tallerthan 11 mm lead to separation downstream of the higher volute cutwaterwall becoming more significant and causing blockages at high flows, alsoresulting in acceleration of fluid and thus localized pressure drop andincreased radial force.

Accordingly, the impeller includes a rotor having a height of at leastone of at least 5 mm, less than 12 mm, between 6 mm and 10 mm and moretypically approximately 10 mm, with the impeller height adding to theoverall height.

Effect of Cutwater Angle CW

The impact of cutwater angle CW will now be described with reference toFIGS. 9L and 9M.

The cutwater angle influences the location within the casing that fluidis either diverted into the volute, or out of the pump through thethroat.

When comparing LV1535, LV1543, LV1527, LV1538 and LV1528, whereby thecutwater angle increases from 37° to 44.2°, it can be observed that ascutwater increases, the minimum resultant force reduces in magnitude andshifts to lower flow rates. Furthermore when analyzing the X and Ycomponent forces, and accounting for variations in other parameters, itis observed that the increasing CW effectively reduces the X forcecomponent away from the cutwater region.

Another way to describe this trend is to consider the distance betweenthe BCD and the OD3. That is, as this value decreases, CW angleincreases, and the forces in the X direction toward the cutwater regionreduces as it is counteracted by forces generated from additionalrecirculating fluid past the cutwater.

When expanding the range of CW angles tested in LV1541, LV1542 andLV1543, a similar trend is observed whereby the minimum resultant forcereduces in magnitude and shifts to lower flows as cutwater increases.Furthermore, as cutwater reduces through 20° to 0°, the X/Y force plotrotates anticlockwise by the same degree. Hence the cutwater angle canbe used to orientate the force over the range of flow rates tocorrespond with the location of bearing stators.

Effect of BCD

The impact of BCD will now be described with reference to FIGS. 9N and9O and FIGS. 9P and 9Q.

When comparing LV1534 an LV1535 in FIGS. 9N and 9O, increasing BCD anddecreasing AR, but maintaining the CW angle, OD3 and A_(th), shifts theminimum resultant radial force point to a lower flow. Regarding theforce components, minimal influence is found on X forces, however Yforces are decreased at low flows, since fluid deceleration at thethroat area is less since more fluid can recirculate between thecutwater and the impeller, thus maintaining velocity and converting lessenergy to pressure. Y forces are increased at high flows in this case,since pressure generation is reduced and thus cannot counteract thecircumferential radial forces acting toward the +Y direction, due to thelower effective throat area caused by excessive blockage downstream ofthe cutwater due to a lower AR and thus larger A_(th) (same H).

When comparing LV1543 and LV1544 in FIGS. 9P and 9Q, where again BCD isincreased and AR is decreased and both CW and OD3 maintained, but thistime A_(th) is reduced, also A_(th) shifts the minimum resultant radialforce point to a lower flow. A similar trend is found with X and Yforces, whereas the larger Y forces at high flows is now simplyattributed to the smaller A_(th) causing additional fluid accelerationretarding pressure generation. This effect can also be observed whencomparing LV1527, LV1528 and LV1535.

Effect of OD3

The impact of OD3 will now be described with reference to FIGS. 9R and9S.

When comparing LV1540 and LV1541, an increase to OD3 as well as A_(th)and AR (same H), whilst maintaining CW angle and BCD, shifts theresulting radial force minimum to a higher flow rate. Regarding theforce components, minimal influence is found on X forces at all flowrates, and Y forces at low flow rates, however Y forces at high flowrates are reduced, due a lower requirement for fluid acceleration, sincethe larger OD3 increases the effective throat area by counteracting theblockage downstream of the cutwater caused by separation. This effectcan also be observed when comparing LV1528 with LV1538 and LV1543.

Effect of CW AND OD3

The impact of cutwater angle CW and outer circle diameter OD3 will nowbe described with reference to FIGS. 9T and 9U.

When comparing LV1543, LV1538 and LV1528, whereby CW is increased as OD3decreases while all other parameters remain equal, it can be observedthat the minimum force point of the resultant radial force shifts tolower flows, as well as reducing in magnitude. Regarding the forcecomponents, there is minimal influence on Y forces at low flows, howeverthe increase in OD3 acts to decrease Y forces at high flows.Additionally, as CW increases, the force in the X direction decreases.This is due in part to a better matching of fluid velocity vectorsinteracting with the cutwater, creating less downstream blockage of theA_(th) due to separation. The larger effective throat area results inincreased fluid deceleration over the entire flow range, and thus raisesthe local pressure near the cutwater, which assists in counteractingunbalanced radial forces in the X direction.

Effect of CW AND BCD

The impact of cutwater angle CW and base circle diameter BCD will now bedescribed with reference to FIGS. 9V and 9W.

When comparing LV1535, LV1527 and LV1528, whereby CW is increased as BCDincreases while all other parameters remain equal, it can be observedthat the minimum force point of the resultant radial force also shiftsto lower flows, as well as reducing in magnitude. As observed earlier,the influence on Y flows is dominated by the variations in BCD, withhigher −Y forces experienced with lower BCDs, while the X forces againreduce as CW increases.

Taking the above into account, for the left pump the cutwater angle istypically at least one of between 0° and 70°, between 30° and 50°,between 40° and 45°, between 35° and 45°, between 45° and 50°, between0° and 60° and approximately 45°. In contrast for the right pump thecutwater angle is typically between 90° and 180°, between 90° and 135°,between 0° and 90°, between 45° and 90°, between 45° and 135°, between60° and 80° and approximately 70°.

Combined Effect

It will be appreciated that the above described variables need to beconsidered in conjunction, and their net combined effect is discussedbelow.

In this regard, effects of different volute configuration on radialforces are shown in FIGS. 9E and 9F. These highlights that the circularvolute (V1) with a throat area in the traditional range for blood pumpsof 50 mm² produces unmanageable radial forces over the desired operatingrange of 3-12 LPM.

Switching to a traditional single volute typical of a conventional bloodpump with CW of 0° and a throat area of 61 mm² designed for a BEP around6-7 LPM (LV1540), radial forces are reduced significantly, andsufficiently below 1.0N over the range of 3-12 LPM. However, the pumpcharacteristic curve is steep (−10 mmHg/LPM), and hydraulic efficiencywhilst acceptable at 25% at BEP, is unacceptably poor at just 10% at 12LPM. These characteristics, whilst arguably suitable for leftventricular assistance, are not appropriate for total ventricularreplacement.

To provide improved performance over the flow range of 3-12 LPM, thethroat area of the single volute was increased to 155 mm², correspondingto a BEP of >10 LPM. To minimize the total radial dimension of thedevice, the cutwater angle was also increased to 37°, creating an AR of1.4. Not only does efficiency increase above 25% for the entire flowrange, peaking at 38%, but the pump HQ curve returns a much flatterresponse (−3 mmHg/LPM). The increase in A_(th) acts to shift the minimumresultant radial force to higher flows, and whilst the minimum radialforce reduced to 0.4N, this occurred at the upper limit of the range at12 LPM, rising to an undesirable 1.5N at 3 LPM.

To reduce the large radial force component at low flows, the voluteconfiguration was modified to increase the BCD from 54 mm to 58 mm,whilst maintaining CW and A_(th) by reducing the AR to 1.0 (LV1534).This reduces low flow radial forces by allowing the fluid deceleratinginto the A_(th) an alternate recirculating pathway between the impellerand the cutwater, thus mitigating the level of deceleration and hencethe conversion to pressure in this region, such that the force in the −Ydirection is reduced. However, this configuration does not reduce the Xforce component. The result is a resultant force profile that has aminimum at 10 LPM, but still raises to 1.3N at the lower 3 LPM range.Furthermore, hydraulic efficiency reduced by 8%.

To further reduce resultant radial forces at low flows whilstmaintaining the same A_(th), CW was increased further to 42.15° and ARincreased back to 1.4 (LV1527), by reducing the H of A_(th). Thisconfiguration shifted the minimum radial force to 8 LPM effectivelyachieving the goal of resultant radial force below 1.0N for the entireflow range of 3-12 LPM, with comparable efficiency to LV535. The largerCW helps to reduce the effective area at the A_(th), hence minimize theamount of fluid deceleration and thus raises localized pressure at thecutwater region, effectively counteracting the unbalanced forces towardthis area. The increase in AR by reduction in throat H has a similareffect.

A further reduction in low flow forces can be achieved by expanding onvalues of the parameters identified above. Whilst again maintainingA_(th), AR, the BC can be increased to 60 mm with a corresponding CWangle to 44.2° (LV1528). This configuration not only reduced the flowrate for which the minimum force occurred, but the magnitude of suchforce was reduced to 0.1N, hence the maximum force over the desired flowrange of 3-12 LPM was just 0.85N. The already observed decrease in Xforce due to cutwater increase and reduction in −Y forces at low flowsdue to the increased BC we responsible for this improved performance atcomparably high efficiency.

Finally, in the case where the device may be operated in pulsatileoutflow mode, the range of flow for which the radial force should remainbelow 1.0N may benefit from a higher limit of at least 15 LPM. Hence,while again maintaining A_(th) and AR, a volute configuration was madewhereby the OD3 was increased from 71 mm to 76 mm with a correspondingdrop in CW to 40°. This increase in OD3 effectively reduced resultantradial forces to below 1.0N from 3-15 PM by reducing radial Y forces athigh flows. However the reduction in CW caused the X forces to increase,and hence low flow forces to increase slightly, which led to a slightdrop in hydraulic efficiency by 5%.

Despite the changes in radial force profile, the performance curves foreach volute configuration with an A_(th)>150 mm² were only marginallyaffected by the volute parameters.

Hence, it can be concluded that for a device operating between flowranges of 3-12 LPM, the volute configuration LV1528 has superiorresultant radial force profile, hydraulic efficiency, and pump curvegradient. Should the flow range need to be expanded to account forpulsatile outflow operation, the LV1543 configuration should beconsidered despite the reduction in efficiency. However increasing CWback to 45° or above can restore this.

Emboli Tolerant Right Pump

Emboli originating from the deep systemic veins may find their way intothe right side of the blood pump. These naturally occurring emboliusually pass through the native right ventricle unimpeded and arefiltered by the pulmonary network. Thus there is a requirement for theright pump to have large area flow paths to enable venous emboli to passthrough the device.

In order to improve outflow pressure sensitivity, the cross-sectionalarea of flow paths through the right pump are maximised. These largepassages reduce the characteristic resistance of the flow path allowingfor the passing of these emboli through the right pump.

In one example, the heart pump includes a flow path having across-sectional area throughout the entire pump that is at least 50 mm²,at least 100 mm², at least 125 mm² and more typically at least 140 mm².This includes inlet and outlet ports, inflow and outflow area of theimpeller, and the volute throat area.

It will also be appreciated, that whilst less critical from theperspective emboli tolerance, the left pump can also include a similarlylarge cross-sectional flow path, in particular having a cross-sectionalarea throughout the entire pump that is at least 50 mm², at least 100mm², at least 125 mm² and more typically at least 140 mm².

Left/Right Shunt Flow

As previously mentioned, the impeller rotor is radially separated fromthe cavity by a distance of between 2-4 mm. For a 50 mm diameter rotor,this leads to a cross-sectional flow path between the rotor and cavityof over 500 mm². Accordingly, in practice, the cross-sectional flow pathbetween the right and left pumps is constrained by a separation betweenthe right hand surface of the rotor and the cavity adjacent the magneticbearing. In one example, this leads to a flow path cross-sectional areaof at least 15 mm², no greater than 50 mm² and typically between 20-50mm², depending on the relative position of the impeller rotor within thecavity.

Under normal flow conditions, the majority of blood flows within eachpump, from the inlet to the outlet, meaning there is minimal leakagebetween the left and right pumps. Any such leakage is from the higherpressure left pump to the low pressure right pump, meaning oxygenatedblood is shunted from the systemic to the pulmonary circulatory systems,which has minimal impact from a physiological perspective. This can alsobe beneficial during normal use as this helps prevent stagnation withinthe pump.

Additionally however the left/right shunt flow path can be utilised whenimplanting a heart pump in a subject. In this regard, during theimplantation of the artificial heart, the circulation is supported by acardiopulmonary bypass machine. In this mode, the lungs and heart arebypassed, and the pumping and oxygenation of the blood is assumed by theexternal pump and oxygenator. Therefore, the lungs are deflated, anddepleted from blood flowing through the arterioles.

Traditionally, after the artificial heart is implanted, the bypassmachine outflow is reduced (termed weaning) and the artificial heart isturned on so that it may provide the additional flow required by thecirculatory system.

However, if the speed of the artificial heart is not sufficient togenerate the same outflow pressure as the bypass machine, the blood willbackflow through the artificial heart and enter the lungs, thusoverloading them with blood and potentially causing edema andirreversible damage.

Therefore, the speed of the device needs to be high enough to generatesufficient pressure and also additional flow to compensate for thereduced flow from the bypass machine during the weaning procedure.However, this speed also results in a sudden return of perfusion to thelungs from the right side of the device.

This sudden reperfusion of de-oxygenated blood at a relatively high flowrate can cause the lung vessels to constrict, thus causing a highresistance state, limiting blood flow back to the left side of theartificial heart, and thus to the rest of the body for a period of time.To avoid this situation, the rate of blood reperfusion to the lungsshould initially be low and then gradually increased.

An example process for implanting a heart pump will now be describedwith reference to FIGS. 13A to 13D, which show the status of the pumpand circulatory parameters from data collected during a trialimplantation process.

In this example, the heart pump is initially connected to the subject'spulmonary and systemic circulatory systems, but with the left pumpoutflow blocked, typically by clamping a cannula or the like. The pumpis activated initially running at a low speed, such as 1000 RPM-1250RPM, with blood being pumped from the right pump outlet, through thelungs, returning to the left pump inlet at a very low rate, typicallylower than 0.5 LPM. As a result, the blood passes through the lungarterioles to collect oxygen, and returns to the left side of the pump.With the left outflow blocked, blood then flows through the left/rightshunt flow path, before being returned to the lungs. In this regard, theleft pump pressure is higher than the right pump pressure, due to thelarger diameter of the impeller vanes on the left side of the rotor,thereby ensuring flow into the right pump.

The speed of the pump is gradually increased in a stepwise fashion,until the speed of the device is approximately 1800 RPM, which allowsthe blood flow rate though the lungs to increase to 2 LPM by creating aleft side pressure of approximately 70 mmHg (that typically provided bycardiopulmonary bypass aortic pressure). This pressure is composed of a10 mmHg left inlet pressure, and a 60 mmHg gradient created by the leftimpeller. The shunt flow rate is typically limited by the size of thecross sectional area of the left/right shunt flow path, which istypically 25 mm² for 2 LPM, but could increase to 50 mm² for 3 LPM-5 LPMof perfusion by moving the rotor axially away from the right side.

In this regard, it will be appreciated that the position of the impellercan be controlled in order to adjust the left/right shunt flow pathcross-section, thereby providing control over the degree of shunt flow.For example, the pump can be finally operated with the impeller moved asfar as possible into the left cavity, to thereby increase the separationbetween the impeller and the cavity housing in the right cavity, tothereby increase the flow path cross-sectional area.

It should be noted that if the left/right shunt flow path has a crosssectional area any smaller than 15 mm², which is typically observed withhydrodynamic bearings, this will result in a flow of less than 1 LPM,and possibly insufficient reperfusion, or a longer time on bypass as thelungs are reperfused.

As this process is performed, the blood passing through the lungsgathers more and more oxygen at each pass, thus helping to ease the lungrestriction and resistance. This continues until the restriction in thelung passes, which typically takes between 5 and 10 mins, at which pointthe aortic cross clamp is removed. Backflow is prevented as the pressurein the pump left outflow already matches that in the aorta created bythe bypass machine, hence the circulating flow through the lungcontinues.

Finally, the pump speed is gradually increased further as the bypassmachine speed is reduced, thus allowing the artificial heart to assumethe entire circulation, to both lung and body.

This procedure essentially prepares the lung circulation for full flowby gradually reperfusing the arterioles.

It is important however that this left/right shunt flow path ispreferably no larger than 50 mm², since during normal operation, theshunt of flow from left to right may be deemed excessive. The left toright outflow ratio from the device should be maintained below1(L):1.5(R), as experienced with clinical ventricular septal defectsthat do not manifest into clinical complications.

Accordingly, the above described approach provides a method of operatinga biventricular heart pump during implantation, the method includingconnecting the heart pump to the pulmonary and systemic circulatorysystems, blocking a left pump outflow to the system circulatory system,operating the pump so that blood flow recirculates through the lungswith blood flow received from the lungs via a left pump inlet beingshunted to a right pump via a left/right shunt flow path so that bloodis supplied to the lungs via a right pump outlet and once the lungs areperfused, unblocking the left pump outflow so that blood flows throughthe pulmonary and systemic circulatory systems.

This approach ensures the lungs are perfused prior to full speedoperation of the device, which in turn assist in preventing lung damage.

Typically the method includes initially operating the pump at arotational speed of between 1000 RPM and 1250 RPM or with a blood flowrate though the lungs of approximately 0.5 LPM. Following this therotational speed of the pump is increased until the rotational speed isapproximately 1800 RPM or the blood flow rate though the lungs increasesto approximately 2 LPM.

Pumping of blood is then performed for between 5 minutes and 10 minutesor until lung resistance reduces, at which point the left pump outflowto the system circulatory system is unblocked.

Improved Inherent Left/Right Flow Balancing

The natural heart continuously balances systemic and pulmonary flowthrough ventricular interdependence and the Frank-Starling mechanism.When a rotary blood pump is used to replace or assist failingventricles, the pump must provide or facilitate this functionality inorder to meet the subject's physiological requirements.

For example, in the case of left ventricular assistance, if the deviceattached to the left ventricle is underperforming, pulmonary venoushypertension results, leading to compromised lung function. If howeverthe left device is over performing, systemic hypertension can result inhigh systemic arterial pressure leading to the risk of haemorrhagicstroke. Conversely, the left pump may also remove excessive fluid fromthe left heart cavity leading to left intermittent or complete chambercollapse and cessation of forward flow.

In the case of biventricular support or replacement, if the pulmonarypump is under performing, systemic venous hypertension results withresultant peripheral oedema, and liver and kidney failure. In addition,the inadequate flow through the pulmonary circulation predisposes tosuction events at the left pump inlet and hemodynamic collapse.Similarly, systemic pump hypo-function results in pulmonaryhypertension, oedema, and lung failure as well as systemic hypotensionand progressive shock. Maintaining flow balance is essential to avoidthese potentially lethal hemodynamic states.

Currently rotary pumps do not automatically provide this functionality,and additional control mechanisms, such as speed control mechanisms arerequired.

The intrinsic ability of all rotary blood pumps to increase or decreaseflow in response to changes in pressure differential is an attractivefeature in that it might allow a rotary pumps configured forbiventricular support to maintain some degree of balance without activepump speed and/or axial position management. Although integrated pumpsare currently under development that use methods to assist this balanceautonomously, the design of their respective left and right impeller cansignificantly expand the balance capability over a wide range ofphysiologic conditions. Furthermore the use of a dual pump necessitatesthat the speeds of the two pumps be precisely set to avoid systemic orpulmonary hypo-function. By choosing two impeller designs with theappropriate intrinsic pressure sensitivity, the demands of relying onthe integrated pump's balancing methods, or indeed adjusting rotationalpump speed of dual pumps moment by moment can be decreased.

The intrinsic pressure sensitivity of a rotary blood pump can bequantified, and an average pressure sensitivity of traditional pumps is0.1 LPM/mmHg, which is roughly half to a third of what is observed inthe ‘unexcited’ normal human heart at a given heart rate. As such, forevery one-millimetre mercury increase in inlet pressure, there is anincrease in pump output of one tenth of a litre per minute, without achange in rotational speed. Stated another way, as the inlet pressure ofone pump starts to rise, as a result of pulmonary-systemic imbalance,that pump will autonomously begin to pump more without adjusting pumpspeed, autonomously bringing the pumps back into balance. By optimizingthis functionality, the requirements for frequent and precise speedchanges can be reduced. Eliminating the need to devise a complex systemfor sensing pump performance and managing speed changes may decreasedevelopmental time, as well as device complexity, cost, and powerconsumption. When used together, an optimized hydraulic system candecrease the demands placed on an active control system and serve asbackup in case of sensor failure or control system failure.

During everyday patient activities, such as changing posture andcoughing, a heart pump is required to alter the relative outflow fromthe left and right pump chambers to keep flows balanced in the face ofvarying vascular resistances.

The shape of the left and right pump curve and their combination(relative left/right design pressure at a given flowrate and speed)dictates the ability for the TAH device to balance flows and thusmaintain atrial pressure between 0-20 mmHg over a range of L/Rresistance combinations.

To achieve the widest range of fluid balance, the right impeller shouldexhibit a flatter pump curve than the left. That is to say, the pressuresensitivity of the right impeller should be higher than the left, aphenomenon observed with the natural heart. The pressure sensitivity ofconventional RBPs (0.05-0.1 LPM/mmHg) however is much lower than that ofthe natural heart (0.2-0.3 LPM/mmHg and up to 3-5 LPM/mmHg withexcitation leading to heart rate changes). Thus it is desired tomaximize the pressure sensitivity of the impellers, with particularlyclose attention to the right impeller.

The design point (ΔP) also significantly affects the range for which thedevice can balance flow whilst maintaining inflow pressures within aspecified range. This is determined by the speed of the left pump toachieve 80 mmHg at 6 LPM and then, with this speed, determining thediameter of the right pump to achieve 20 mmHg at 6 LPM. This ratio ofdelta pressure (eg: 80 mmHg/20 mmHg) created by each side of the devicealso influences this range. A relatively stronger right pump to leftwill mean that higher pulmonary vascular resistance (PVR) can beaccommodated prior to the transition of left atrial pressure below 0mmHg. However the propensity for left atrial pressure (LAP) to riseabove 20 mmHg increases at high systemic vascular resistance (SVR) andlow pulmonary vascular resistance (PVR), although this can be accountedfor by axial translation of the impeller to increase the performance ofthe left pump, or indeed by increasing impeller speed to increase bothpump's performance.

The ability to axially translate the impeller, and maximizing itsability alter pressure for this movement (ie axial pressuresensitivity), creates alternate ratios of left/right design pressure,which widens the range of acceptable resistance combinations that may beencountered due to patient-to-patient resting variability, as well asthose resistances presented during transient daily activities. Shiftingthe rotor toward the right chamber opens up the clearance above the leftimpeller, thus reducing outflow from the left side of the device. Thisproduces a momentary shift in relative left and right chamber hydraulicperformance, assisting to balance circulatory flows.

A typical ratio of resistances for a healthy patient at rest isSVR=1433±229 dyne·s·cm⁻⁵ and PVR=85±33 dyne·s·cm⁻⁵. A typical ratio ofresistances for a heart failure patient (on medication) at rest isSVR=1127±390 dyne·s·cm⁻⁵ and PVR=233±119 dyne·s·cm⁻⁵.

A healthy patient might benefit from a L:R delta pressure ratio of5.33:1 (LEFT 80 mmHg:15 mmHg RIGHT) whilst a HF patient with a rotaryblood pump might benefit from a ratio of 4:1 (LEFT 80 mmHG:20 mmHgRIGHT) (ie relatively stronger right pump) to accommodate for relativelyhigher PVR and clinically preferred lower arterial pressure.

During everyday activities, these ranges may alter transiently. Whilstcoughing or Valsalva, PVR alone can rise significantly up to +400% for ashort period (up to 425 dyne·s·cm⁻⁵). During a postural change, SVRalone can drop by −40% and then rise by +60% than settle to +30%. Duringa transition to exercise, SVR and PVR can both drop by up to −20 to −50%with a corresponding change in flow of >5 LPM.

With a left pump gradient of 50%, this change in flow of 5 LPM wouldresult in a drop in aortic pressure of just 10 mmHg With a right pumpgradient of 100%, a drop in pulmonary pressure of just 5 mmHg wouldresult.

Since, during exercise, the skeletal muscle pump returns blood to theright atrium, and raises pressure (sometimes by 10 mmHg), these minordrops in left/right pump delta pressure could potentially reduce theincidence of arterial hypotension and associated syncope and dizziness.

An example of the capabilities of pumps having different pump curves isillustrated in FIG. 10A, with unbalanced (depicted as when inletpressure reduces below 0 mmHg or rises above 20 mmHg) areas shaded grey.The resistance box composition is explained in the following by theexample of the steep/steep combination.

With increasing SVR more volume is pooled in the left atrium and itspressure rises. At the same time right atrial pressure decreases so thatthe pressure difference between both atria (PDI) increases. If SVR keepsincreasing, PDI will be above or below its predefined limits.Furthermore, total flow (Qs) will drop below a safe level (defined as 2LPM). Consequently, any SVR-PVR combination below the line 1001 (LAP>20mmHg), above the line 1001 (LAP<OmmHG), or to the right of the line 1002(Qs<2 LPM) is not considered in balance.

Additionally, the pump's left and right relative design pressure (deltapressure from measured from inlet to outlet at a given flow rate androtational speed) affects the range of patient states for which theeffective fluid balance can be maintained at a given rotational speed.This design pressure for the left pump is reached for a given pumpdesign at a determined speed, and with this speed, the design pressureof the right pump is determined by the selection of the right pumpdesign parameters (such as impeller vane outer diameter). The ratio ofthe resulting left and right design pressures at a given flow rate andspeed is defined as the ‘left/right design pressure ratio’. Examples,are shown in FIGS. 10B and 10C, which show the effect of left/rightpressure design for pumps with left: right pressures of 80 mmHg:15 mmHgand 80 mmHg:20 mmHg at 2050 RPM, respectively. Here it can be seen thatthe smaller ratio (4:1) than (5.33:1) can lead to a larger window ofbalanced resistance conditions.

Furthermore, shifting the rotor axially within the pumping cavity canenhance the effect of alterations in left/right relative design pressureand axial pressure sensitivity (APS) of a RBP to help autonomouslybalance left and right outflows by altering the relative pumpingcapacity of each pump. For example, moving the rotor toward the rightchamber opens up the clearance above the left impeller, thus reducingoutflow and pressure from the left side of the device. This produces amomentary shift in relative left and right chamber hydraulic performance(altering the left/right design pressure ratio), assisting to balance alarger range of circulatory flows in a shorter period of time. Thisoutcome can be likened to a change in relative rotational speed of theleft and right impellers, however in this case speed is fixed. Examplesof the effect of axial movement on balance range for different pumps areshown in FIGS. 10D and 10E, for cases of no axial movement and 200 μmmovement that produces an APS of 60 mmHg/mm respectively. The axialmovement increases the range of balanced conditions, effectively byaltering the left/right design pressure ratio from 4:1 to 3.25:1(rightward movement) and to 4.75:1 (leftward movement). The dottedcircles represent hemodynamic data recovered from heart failure patientswith previously implanted total artificial hearts.

It will be appreciated from this that when creating such heart pumps,the particular design of the pump can have a major impact on the pumpperformance curve and thus outflow pressure sensitivity of the heartpump. This affects the ability of the pump to pump blood at differentflow rates at a given rotational speed, depending on the physiologicalrequirements of the subject to which the pump is fitted.

Considering that in the TAH application, the left and right impellersare attached to a common rotating hub spinning one RPM, the deltapressure ratio is most influenced by the relative outer diameter of theleft and right impeller vanes and the relative efficiency of each pump.Thus to reduce the left/right design pressure ratio generate wouldeither require a) a larger diameter right impeller, or b) a relativelyless efficient left impeller.

A larger diameter right impeller reduces area available for the magneticbearing system to target, and as such would reduce its force capacity.Alternatively, a less efficient left impeller may be created by alteringthe parameters of the impeller design, which effectively reduces thehydraulic efficiency and thus pressure generation for a given rotationalspeed and flow rate.

The effect of altering the right impeller diameter from 25 mm to 24 mmwas to reduce design pressure by 2 mmHg at 5 LPM, down from 23.5 mmHg to21.5 mmHg, as shown in FIG. 10F. The smaller diameter right impellercreates a larger gap (2 mm on the radius) between the outercircumference of the right impeller vanes and the base circle diameterof the stationary housing. Thus larger radial forces can be accommodatedbefore potential touchdown. The smaller diameter also matches theselected left impellers to create a pressure ratio of 4:1 at aleft/right vane tip gap of 300 μm.

Consideration however must be given to the effect altering pumpparameters has on OPS and APS when meeting the left/right pressure ratioobjectives, As seen in FIG. 10A, this is of particular importance forthe right pump, whereby the flattest pump curve gradient (less negative)is desired. The left pump gradient is more forgiving, however too steepa gradient will limit the maximum outflow and pressure when systemicresistance drops (during transitions to exercise or upright posture) fora given rotational speed, potentially limiting patient quality of life.

As described in Stepanoff 1957, the pump performance curve gradient andthus OPS is linked heavily to the theoretical Euler head (dictated byselection of inlet and outlet vane angles), the fluid friction lossesthrough the device (determined by the cross-sectional areas through thefluid path), hydraulic losses (determined in part by the mismatch offlow velocities and rotating/stationary vanes entering and exiting thepump) and recirculation losses (determined by the mismatch of flow rate,velocity and fluid path cross-sectional area). These parameters alsoinfluence the APS, with additional parameters such as vane height,angle, width and number having particular importance.

Generally speaking, improving both OPS and APS are competing objectives,and as such an optimum must be sought.

To obtain the flattest pump curve, pressure generation from the pumpshould be reduced at low flow and increased at high flow. This mayhowever also come at the consequence of lower overall pressuregeneration and efficiency. For the right impeller, this means the speedwould need to be higher to meet a certain design pressure, which meansthe left impeller would need to be even less efficient to maintain thetargeted left/right design pressure ratio.

The trade of efficiency to create suitable left/right pump curveperformance gradients however is considered important to provide correctbalancing, which goes against conventional wisdom when designing bloodpumps.

A weaker left pump can be achieved by choking forward flow with smallerfluid path areas, or utilizing a backward swept vane. These cause thepump curve to be steep and thus reduces outflow pressure sensitivity,however improve axial pressure sensitivity.

Alternatively, it was found that increasing the impeller inner eyediameter ID1 rapidly drops pressure from shutoff at low flows, but thenmaintains a flatter curve at higher flows. Thus a weaker impeller can becreated without compromising outflow pressure sensitivity.

Maximizing the axial pressure sensitivity (particularly from the leftimpeller) can be influenced by altering the vane outlet angle (largesteffect) and vane number and vane thickness/height: clearance ratio.

As described, flow path areas tend to influence the resistance of fluidto flow through the pump and thus outflow pressure sensitivity, withlarger areas generally creating flatter pump curves.

However larger areas also tend to assist with device biocompatibility.Larger areas between the rotating impeller and the stationary casingresult in smaller shear stresses in this region and thus reduced redblood cell lysis (haemolysis). However larger flow path areas are alsosuggested to reduce the incidence of vonWillebrand factor (vWF)degradation. Degradation of this molecule can lead to impaired abilityfor blood to clot, thus raising the risk of bleeding complications.

vWF degradation is often observed in patients implanted with rotaryblood pumps. It is also observed in patients who suffer from aorticvalve stenosis. In these situations, the peak blood velocity (10-20 LPM)ejected from the (often weakened) heart transverses through a narrowedorifice in the order of 10-12 mm in diameter (78.5-113.04 mm²).Furthermore in the case of blood pumps, the smallest flow path area forwhich the entire flow rate must transverse may reduce to 60 mm² orlower.

This vWF degradation in both scenarios is reversed after hearttransplantation, or in the case of valve stenosis, with the replacementof a mechanical heart valve (areas of 314-706 mm²). However some vWFdegradation remains when an undersized (smaller area) valve isimplanted.

These observations suggest that raising the minimum blood flow areathrough the device to values greater than 60 mm² and even greater than110 mm² may assist in reducing the severity of vWF degradation.

In the above described heart pump, the right pump curve is extremelyflat (typically −1 mmHg/LPM), whilst the left pump curve is slightlysteeper (typically −2 to −4 mmHg/LPM), as shown in FIG. 10G, helpingprovide automatic flow balancing.

Additionally, since no radial journal bearing was used to further limitthis shunt flow, total axial clearance was reduced from 1.2 mm to 0.7 mmto reduce the washout clearance. It was further reduced to 0.6 mm tominimize leakage, since the majority of the impeller axial pressuresensitivity (APS) occurs in the initial 0.5 mm of movement, as shown inFIG. 10H. The operating range of the impeller is restricted to +/−200 μmwhen operating in a total 0.6 mm clearance, so as to not allow theimpeller to come with 100 μm of the stationary casing to assist withhemocompatibility. However the rotor may be able to come within 50 μm ofthe stationary casing at critical times to assist with flow balancing,effectively expanding the range of movement to +/−250 μm. It would beappreciated that these limits from the casing are fixed, resulting in alarger operating range as total clearance gap increases.

Accordingly, the above described blood pump provides improvedcharacteristics including improved outflow pressure sensitivity,improved axial pressure sensitivity, reduced radial hydraulic forcesover the desired flow range, emboli tolerance and improved inherentleft/right flow balancing. These are achieved through a combination ofpump parameters, including increasing the cross-sectional areas of flowpaths (inlet and outlet port diameters) through the pump, reducingrecirculation by increasing impeller vane height and/or thickness andadding secondary vanes, using a larger than normal throat area toprovide a BEP at a higher flow rate, and selecting a volute design tominimize radial forces over a desired flow range.

Some example impeller configurations optimized for specific scenarioswill now be described with reference to FIGS. 11A to 11L. Parameters forthe respective impellers are shown in Table 2 below.

TABLE 2 OPS Thrust APS Left Left Left OPS/APS OPS Left Right TAH TAH TAHLeft TAH VAD TAH deltaP@2100 RPM 80 80 74 81.5 80@1680 RPM 20 @5 LPM L/RPressure ratio (3-7)-4- (3.5)-4- (3)-3.6- (3.5)-4.1- N/A N/A (4.3):1(4.5):1 (4.75):1 (4.7):1 Primary eye 30 25 25 30 25 16 diameter ID1 (mm)Secondary eye 35 N/A N/A N/A N/A 19 diameter ID2 (mm) Rotor Diameter 5050 50 50 50 24 OD (mm) Number of primary 4 4 4 8 10 4 vanes Number of 40 0 0 0 4 secondary vanes Vane outlet angle 80 80 20 40 80 90 (°) Vaneinlet angle 84 84 64 64 84 90 (°) Vane height (mm) 2.0 2.0 2.0 2.0 4.017.5 Vane width (mm) 7.5 18 15 15 1.0 1.5 APS (mmHG) 13 20 30 25 2 N/AOPS −1.8 −3.2 −4 −2.6 −0.1 to −0.2 −0.3 (mmHg/LPM) 55% 31.25% 25% 38%500-1000% 333%

The OPS impeller is shown in FIG. 11A, with the performance curve beingshown in FIG. 11B. The OPS impeller demonstrates characteristics foroptimal OPS whilst still having some APS and creating the targeted L/Rpressure ratio, and is characterized by the use of primary and secondaryvanes with a large eye diameter and high inlet/outlet angle.

The thrust impeller is shown in FIG. 11C, with the performance curvebeing shown in FIG. 11D. The thrust impeller demonstratescharacteristics for providing a backup thrust (hydrodynamic) axialbearing, and is characterized by the use of vanes having a thick outeredge to provide a high end surface area. The profile of the vanedictates that the leading edge is approx. 50-100 μm lower in thedirection of the axis of rotation than the trailing edge of the vane.

The APS impeller is shown in FIG. 11E, with the performance curve beingshown in FIG. 11F. The APS impeller demonstrates characteristics foroptimal APS, and is characterized by highly curved vanes with a smalloutlet angle.

The OPS/APS impeller is shown in FIG. 11G, with the performance curvebeing shown in FIG. 11H. The OPS/APS impeller demonstrates a balancebetween OPS and APS making it the preferred impeller for TAHapplications, and is characterized by the use of primary curved vaneswith a medium-to-small outlet angle.

The OPS VAD impeller is shown in FIG. 11I, with the performance curvebeing shown in FIG. 11J. The OPS VAD impeller demonstrates extremelyhigh OPS with minimal APS making it ideal for use in single VADscenarios, and is characterized by the use of a large number ofrelatively high, thin vanes having large inlet/outlet angles.

The RVAD impeller is shown in FIG. 11K, with the performance curve beingshown in FIG. 11L. The RVAD impeller demonstrates high OPS with minimalAPS and lower flow rates making it suitable for as an RVAD in TAHapplications, and is characterized by the use of a number of extremelyhigh primary and second vanes having steep inlet/outlet angles.

An example of a single VAD heart pump will now be described withreference to FIGS. 12A to 12F.

In this example, the heart pump 1200 includes a housing 1210 defining acavity 1215. The housing can be of any suitable form but typicallyincludes a main body, and left and right end caps which connect to themain body. The housing can be made of any suitable biocompatiblematerial, and can be made of titanium, a polymer or the like.

The housing 1210 includes an inlet 1211, for connection to the leftatrium/pulmonary vein or right atrium/vena cava, or left or rightventricle, and an outlet 1212 for connection to the aorta or pulmonaryartery, respectively.

The heart pump 1200 includes an impeller 1220 provided within the cavity1215. The impeller 1220 includes a rotor 1221 having vanes mountedthereon for urging fluid from the inlet 1211 to the outlet 1212 uponrotation of the impeller 1220. In this example, as the heart pump 1200is a single ventricular assist device, the impeller includes a singleset of vanes 1222 for urging fluid from the inlet 1211 to the outlet1212. In this example, the vanes 1222 have a configuration similar tothat described above with respect to FIGS. 11I and 11J, and these willnot therefore be described in further detail, although it will beappreciated that other suitable vane configurations can be used. Theimpeller can also include an aperture 1224 extending therethrough toallow blood to flow around the rear surface of the impeller and therebyprevent stagnation and clotting of blood within the heart pump.Furthermore, the use of a magnetic bearing in this region allows forblood gaps in excess of 200-300 μm and up to 500 μm, which can bothreduces shear stress and thus red cell lysis, as well as promote greaterrates of washout flow than otherwise anticipated in gaps created byhydrodynamic bearings.

The heart pump 1200 further includes a drive 1230 that rotates theimpeller 1220 within the cavity 1215. The drive 1230 can be of anyappropriate form but typically includes a number of coils 1231, eachwound on a respective stator 1232, supported by a mounting 1233,allowing the drive 1230 to be coupled to the housing 1210. The drivecooperates with magnetic material 1234 mounted in the rotor 1221, withthe magnetic material being in the form of a number of circumferentiallyspaced permanent drive magnets arranged proximate an outercircumferential edge of the rotor 1221. In one example, the coils 1231and stators 1232 are wedge shaped and circumferentially spaced aroundthe mounting 1233, so as to provide twelve electromagnets radiallyaligned with the drive magnets 1234 in the rotor 1221, to therebymaximise a degree of magnetic coupling between the magnets in the rotor1221 and the drive 1230.

The heart pump 1200 can further include a magnetic bearing 1240including at least one bearing coil 1241 that controls an axial positionof the impeller within the cavity 1215. In one particular example, shownin more detail in FIG. 12E, the magnetic bearing includes three bearingcoils 1241, each of which is mounted on a first leg 1242.1 of respectiveU-shaped stators 1242, with a second leg 1242.2 being positionedradially inwardly of the first leg 1242.1. The stators 1242 are mountedto or integrally formed with a support 1243 and circumferentially spaced120° apart around the housing so that the first and second legs 1242.1,1242.2 align with respective magnetic material, such as bearing magnets1244, 1245 within the impeller 1220, allowing an axial position of theimpeller 1220 to be controlled.

In one particular example, the bearing rotor assembly includesferromagnetic core target 1244 mounted in the rotor, proximate an outercircumferential edge of the rotor 1221, and a permanent bearing magnetor ferromagnetic material 1245 mounted radially inwardly of the firstferromagnetic core target 1244, so that the ferromagnetic core targetand bearing magnets 1244, 1245 align with respective legs 1242.1, 1242.2of the stators 1242. The ferromagnetic core target can be replaced witha second permanent magnet. However, the use of a magnetic bearing maynot be required and can be replaced by a static physical bearing orhydrodynamic bearing, or the like.

In this example, the drive 1230 and magnetic bearing 1240 are mounted atopposing ends of the housing 1210 so that the drive and bearing 1230,1240 are provided proximate opposing surfaces of the rotor 1221 as shownfor example in FIG. 12B. In the current example the drive 1230 ismounted adjacent the side of the impeller 1220 that includes vanes so asto maximise the blood gap between the rotor, vanes and the casing. Thatis to say, only the vane tips are in closer proximity to the casing,however this blood gap can still be in the order of 200-300 μm, and upto 500 μm. Additionally, bearing and drive are configured so that themagnetic forces inherent between the drive 1230 and impeller 1220, andbetween the magnetic bearing 1240 and impeller 1220 and the hydraulicforces on the impeller 1220 define a balance position within the cavityunder conditions of normal flow. This minimises the bearing currentrequired to maintain the position of the impeller 1220 within the cavityunder nominal flow conditions.

It will be appreciated as in the previous example, the apparatus canfurther include a controller, and otherwise functions largely aspreviously described, and hence will not be described in further detail.

Some benefits of these arrangements include:

-   -   The ability to maintain passive radial suspension with just an        axial magnetic bearing from 0-15 LPM without the need of journal        bearing or other radial support bearings is achieved.    -   An increase in device outflow of up to 40% for a TAH and up to        50-100% for a VAD, with just a 15% reduction in arterial        pressure during a transition from rest to exercise without a        change in rotational speed.    -   Improved left impeller design to allow greater axial pressure        sensitivity.    -   Improving the pulse pressure produced during pulsatile operation        since smaller rotor RPM changes are required for larger changes        in pressure.    -   Improved biocompatibility by removing the need for a journal        bearing by enlarging the radial gap to >2 mm    -   Allowing the native heart to transmit pressure and flow        pulsatility (VAD only).    -   Reduction in diastolic pressure thus allowing the arterioles to        rest which may reduce the incidence of aneurysm or GI bleed (VAD        only).

The above described arrangement can be employed in wide range ofcircumstances and in different pump configurations. For example, thiscan be used when one or two pumps are used to provide assistance orreplacement of the left or right ventricles, including in a TAH, whentwo rotary pumps to provide complete replacement of the native heart, inan LVAD/RVAD, when a single rotary pump is used to provide assistance toeither the left or right ventricles, or in a BiVAD, when two rotarypumps to provide assistance to either the left or right ventricles.

The heart pump can be used with a controller and control process thatuses an active magnetic bearing in conjunction with a zero powercontroller that controls the position of the rotor in response to achange of magnetic bearing current, or that uses speed control based onimpeller position, for example in response to a perturbation inflow/bearing operation to provide an additional degree of control overflow.

Throughout this specification and claims which follow, unless thecontext requires otherwise, the word “comprise”, and variations such as“comprises” or “comprising”, will be understood to imply the inclusionof a stated integer or group of integers or steps but not the exclusionof any other integer or group of integers. As used herein and unlessotherwise stated, the term “approximately” means±20%.

Whilst reference is made to absolute values of dimensions in the pump,it will be appreciated that these are for the purpose of illustrating aparticular preferred embodiment. However, the pump is largely scalable,meaning that interrelated dimensions could be defined as percentagesrelative to one another, as long as these meet minimum requirements,such as the minimum flow part cross-sectional area requirements. Forexample, impeller dimensions for the left and right pumps could bedefined relative to one another, so the left hand dimensions are a setpercentage of the right hand dimensions. Similarly, the dimensions ofthe impeller, including the vane and rotor sizes could be definedrelative to housing and cavity dimensions.

Persons skilled in the art will appreciate that numerous variations andmodifications will become apparent. All such variations andmodifications which become apparent to persons skilled in the art,should be considered to fall within the spirit and scope that theinvention broadly appearing before described, including equivalents.

What is claimed is: 1) A heart pump including: a) a housing forming acavity including: i) at least one inlet aligned with an axis of thecavity; and, ii) at least one outlet provided in a circumferential outerwall of the cavity; b) an impeller provided within the cavity, theimpeller including vanes for urging fluid from the inlet to the outlet;and, c) a drive for rotating the impeller in the cavity and wherein aflow path through the entire pump has a minimal cross-sectional area ofat least one of: i) at least 50 mm²; ii) at least 75 mm²; iii) at least100 mm²; iv) at least 125 mm²; v) at least 140 mm²; vi) at least 150mm²; vii) at least 200 mm²; and, viii) up to 300 mm². 2) A pumpaccording to claim 1, wherein the pump at least one of: a) has aperformance curve having a gradient of less than −20% over a definedflow range such that a change in pressure of 10 mmHg across the pumpcauses a change in flow rate of at least 2 LPM, the defined flow rangebeing between at least one of: i) 5 LPM and 8 LPM; ii) 3 LPM and 12 LPM;and, iii) 3 LPM and 15 LPM; and, b) generates a pressure head that is atleast one of: i) for a pump that provides at least partial leftventricular function: (1) between 60 mmHg and 100 mmHg at 6 LPM; (2)between 70 mmHg and 90 mmHg at 6 LPM; and, (3) approximately 80 mmHg at6 LPM; and, ii) for a pump that provides at least partial rightventricular function: (1) between 10 mmHg and 30 mmHg at 6 LPM; (2)between 15 mmHg and 25 mmHg at 6 LPM; and, (3) approximately 20 mmHg at6 LPM. 3) A heart pump according to claim 1, wherein the heart pump isconfigured to provide at least partial left ventricular function. 4) Aheart pump according to claim 3, wherein at least one of: a) the heartpump has a pump performance curve having a gradient less than at leastone of: i) −25%; ii) −30%; iii) −35%; iv) −40%; v) −100% vi) −200%; and,vii) −500%; and, b) an axial position of the impeller within the cavitycontrols in part a flow of fluid from the inlet to the outlet, andwherein a change in axial position of 200 μm causes at least one of: i)a change in flow rate of at least one of: (1) at least 1 LPM; (2) atleast 2 LPM; (3) less than 4 LPM; and, (4) between 2 LPM and 3 LPM; and,ii) a change in flow pressure of at least one of: (1) at least 5 mmHg;(2) at least 10 mmHg; (3) at least 15 mmHg; (4) at least 20 mmHg; (5) atleast 25 mmHg; (6) at least 30 mmHg; (7) at least 35 mmHg; and, iii) atleast 40 mmHg. 5) A heart pump according to claim 1, wherein the outletat least one of: a) has a throat area of at least one of: i) at least 60mm²; ii) at least 80 mm²; iii) at least 120 mm²; iv) between 60 mm² and250 mm²; v) between 120 mm² and 160 mm²; vi) between 140 mm² and 160mm²; vii) between 140 mm² and 250 mm²; viii) between 130 mm² and 150mm²; ix) approximately 140 mm²; and, x) approximately 150 mm²; b) has asubstantially rectangular cross-sectional shape and a width to heightaspect ratio of at least one of: i) between 1:2 and 2:1; ii) between 1:1and 2:1; iii) between 1:1 and 1.8:1; iv) between 1.1:1 and 1.6:1; and,v) approximately 1.4:1; and, c) defines a cutwater angle of at least oneof: i) between 0° and 70°; ii) between 30° and 50°; iii) between 40° and45°; iv) between 35° and 45°; v) between 0° and 60°; and vi)approximately 40°. 6) A heart pump according to claim 1, wherein theimpeller has at least one of: a) a vane height of at least one of: i) atleast 1.5 mm; ii) less than 5 mm; iii) between 1.5 mm and 3 mm; iv)between 1.7 mm and 2.3 mm; v) between 1.8 mm and 2.2 mm; and, vi)between 1.9 mm and 2.1 mm; and, vii) approximately 2 mm; and, b) a vaneinlet angle of at least one of: i) less than 90°; ii) greater than 60°;iii) between 70° and 90°; iv) between 82° and 86°; and, v) approximately84°; and, c) a vane outlet angle of at least one of: i) less than 60°;ii) greater than 20°; iii) between 30° and 50°; iv) between 45° and 50°;v) between 35° and 45°; vi) between 38° and 42°; and, vii) approximately45°. 7) A heart pump according to claim 1, wherein the impeller includesat least one of: a) a number of primary vanes, the primary vanes havingat least one of: i) an inner diameter of at least one of: (1) largerthan a diameter of an inflow port; (2) at least 10 mm; (3) less than 40mm; (4) between 20 mm and 40 mm; (5) between 25 mm and 35 mm; and, (6)approximately 25-30 mm; and, ii) an outer thickness of at least one of:(1) at least 5 mm; (2) less than 20 mm; (3) between 6 mm and 15 mm; (4)between 7 mm and 8 mm; and, (5) approximately 7.5 mm; b) a number ofsecondary vanes, the secondary vanes having an inner diameter of atleast one of: i) at least 20 mm; ii) less than 40 mm; iii) between 30 mmand 40 mm; and, iv) approximately 35 mm; and, c) an outer vane diameterof at least one of: i) at least 20 mm; ii) less than 60 mm; iii) between45 mm and 55 mm; iv) between 48 mm and 52 mm; and, v) approximately 50mm; and, d) an equal number of primary and secondary vanes; e) at leastthree primary and secondary vanes; f) less than six primary andsecondary vanes; and, g) four primary and four secondary vanes. 8) Aheart pump according to claim 1, wherein in a region of an outlet volutethe cavity has at least one of: a) a base circle diameter of at leastone of: i) at least 40 mm; ii) at least 50 mm; iii) less than 100 mm;iv) less than 80 mm; v) between 50 mm and 74 mm; vi) between 54 mm and64 mm; and, vii) approximately 60 mm; and, b) an outer wall diameter ofat least one of: i) at least 50 mm; ii) less than 100 mm; iii) less than80 mm; iv) between 50 mm and 80 mm; v) between 65 mm and 76 mm; and, vi)approximately 71 mm. 9) A heart pump according to claim 8, wherein overthe defined flow range the volute generates a maximum radial force ofless than at least one of 1.2N, 1.0N and 0.85N, and wherein the definedflow range is at least one of: a) at least 5 LPM to 8 LPM; and, b) 3 LPMto 12 LPM c) 3 LPM to 15 LPM. 10) A heart pump according to claim 1,wherein the heart pump is configured to provide at least partial rightventricular function. 11) A heart pump according to claim 1, wherein atleast one of: a) the heart pump has a pump performance curve having agradient less than at least one of: i) −30%; ii) −35%; iii) −40%; iv)−75%; v) −100%; and, vi) −150%; and, b) an axial position of theimpeller within the cavity controls in part a flow of fluid from theinlet to the outlet, and wherein a change in axial position of 200 μmcauses at least one of: i) a change in flow rate of at least one of: (1)at least 0.2 LPM; (2) at least 0.5 LPM; (3) less than 2 LPM; and, (4)between 0.5 LPM and 1.5 LPM; and, ii) a change in flow pressure of atleast one of: (1) at least 1 mmHg; (2) at least 2 mmHg; (3) at least 5mmHg; and, (4) at least 10 mmHg. 12) A heart pump according to claim 1,wherein the outlet at least one of: a) has a throat area of at least oneof: i) at least 100 mm²; ii) at least 130 mm²; iii) between 130 mm² and250 mm²; iv) between 130 mm² and 230 mm²; v) between 170 mm² and 210mm²; vi) between 140 mm² and 200 mm²; vii) between 140 mm² and 210 mm²;viii) between 150 mm² and 200 mm². ix) approximately 233 mm²; x)approximately 175 mm²; and, xi) approximately 150 mm²; b) has asubstantially rectangular cross-sectional shape outlet and a width toheight aspect ratio of at least one of: i) between 1:3 and 1:1; and, ii)approximately 0.45-0.65:1; and, iii) a width of between 8 mm and 12 mm;and, c) defines a cutwater angle that is at least one of: i) between 90°and 180°; ii) between 90° and 135°; iii) between 0° and 90°; iv) between45° and 90°; v) between 45° and 135°; vi) between 60° and 80°; and vii)approximately 70°. 13) A heart pump according to claim 1, wherein theimpeller has at least one of: a) a vane height of at least one of: i) atleast 10 mm; ii) less than 30 mm; iii) between 10 mm and 25 mm; iv)between 15 mm and 20 mm; v) between 17 mm and 18 mm; and, vi)approximately 17.5 mm; b) a vane inlet angle of at least one of: i)greater than 60°; ii) less than 115°; iii) between 80° and 100°; and,iv) approximately 90°; and, c) a vane outlet angle of at least one of:i) greater than 60°; ii) less than 115°; iii) between 80° and 100°; iv)approximately 72° and, v) approximately 90°. 14) A heart pump accordingto claim 1, wherein the impeller includes: a) a number of primary vanes,the primary vanes having at least one of: i) an inner diameter of atleast one of: (1) at least 10 mm; (2) less than 25 mm; (3) between 10 mmand 20 mm; (4) between 14 mm and 18 mm; and, (5) 16 mm; ii) a thicknessof at least one of: (1) at least 0.5 mm; (2) less than 3.0 mm; (3)between 0.75 mm and 2.5 mm; and, (4) 1.5 mm; and, iii) a filleted edgeof between 0.25 mm and 1.14 mm. b) a number of secondary vanes, thesecondary vanes having an inner diameter of at least one of: i) at least10 mm; ii) less than 25 mm; iii) between 15 mm and 25 mm; iv) between 18mm and 20 mm; and, v) approximately 19 mm; and, c) an outer vanediameter of at least one of: i) at least 15 mm; ii) less than 40 mm;iii) between 20 mm and 30 mm; iv) between 22 mm and 27 mm; and, v)approximately 24 mm; and vi) approximately 25 mm; d) an equal number ofprimary and secondary vanes; e) between three and five primary vanes; f)four primary vanes; g) between three and six secondary vanes; h) foursecondary vanes; and, i) four primary vanes and four secondary vanes.15) A heart pump according to claim 1, wherein at least one of: a) theinlet has a diameter of at least one of: i) at least 10 mm; ii) at least15 mm; iii) less than 30 mm; iv) less than 25 mm; v) between 18 mm and22 mm; and, vi) approximately 19 mm to 20 mm; and, b) the cavity has adiameter of a least one of: i) at least 20 mm; ii) at least 25 mm; iii)less than 40 mm; iv) less than 30 mm; v) between 27 mm and 29 mm; and,vi) approximately 28 mm. 16) A heart pump according to claim 1, whereinthe impeller includes a rotor having at least one of: a) a height of atleast one of: i) at least 5 mm; ii) less than 15 mm; iii) between 6 mmand 13 mm; and, iv) between 8 mm and 11 mm; and, v) approximately 10 mm;and, b) an outer circumferential wall spaced from an inner cavity wallby at least one of: i) an average distance of at least 2 mm; ii) anaverage distance of less than 8 mm; iii) an average distance of lessthan 5 mm; and, iv) an average distance of approximately 4 mm. 17) Aheart pump according to claim 1, wherein: a) the impeller includes firstand second sets of vanes provided on a rotor body, the rotor beingpositioned within the cavity to define: i) a first cavity portion havinga first inlet and a first outlet, the first set of vanes being providedwithin the first cavity portion so as to define a first pump thatprovides at least partial left ventricular function; and, ii) a secondcavity portion having a second inlet and a second outlet, the second setof vanes being provided within the second cavity portion so as to definea second pump that provides at least partial right ventricular functionand wherein at least one of: (1) the axial position of the impellerdetermines a separation between each set of vanes and a respectivehousing surface, the separation being used to control the fluid flowsfrom the inlets to the outlets; and, (2) the first and second pumps haverespective pump performance curve having different gradients so that achange in rotational speed of the pump causes a change in the relativeflows of the first and second pumps; b) the drive is positioned at afirst end of the cavity and includes: i) a number of circumferentiallyspaced permanent magnets mounted in the rotor of the impeller, adjacentmagnets having opposing polarities; and, ii) at least one drive coilthat in use generates a magnetic field that cooperates with the magneticmaterial allowing the impeller to be rotated; c) the pump includes amagnetic bearing for controlling an axial position of the impellerwithin the cavity and wherein the magnetic bearing is positioned at asecond end of the cavity and includes: i) first and second annularmagnetic bearing members mounted within and proximate a face of therotor, the first magnetic bearing member being outwardly of the secondmagnetic bearing member; ii) a number of circumferentially spacedsubstantially U-shaped bearing stators mounted in the housing proximatea second end of the cavity, each U-shaped bearing stator having firstand second bearing stator legs substantially radially aligned with thefirst and second magnetic bearing members respectively; and, iii) atleast one bearing coil on each bearing stator that generates a magneticfield that cooperates with the magnetic bearing members to thereby atleast one of: (1) control an axial position of the impeller; and, (2) atleast partially restrain radial movement of the impeller. 18) A heartpump according to claim 17, wherein the first and second pumps have atleast one of: a) a design pressure ratio at 6 LPM of 3.5-4.5:1; b) anaxial pressure sensitivity of at least one of: i) at least 20 mmHg/mm;and, ii) approximately 60 mmHg/mm; and, c) a change in design pressureratio at 6 LPM of 3.25-4.75:1 19) A heart pump according to claim 1,wherein the housing and impeller cooperate to provide a shunt flow pathbetween the first and second cavity portions, the shunt flow path havinga cross-sectional area that is at least one of: a) at least 15 mm²; b)no greater than 50 mm²; c) between 20-50 mm²; d) approximately 25 mm²;and, e) adjustable by controlling an axial position of the impellerwithin the cavity 20) A heart pump including: a) a housing forming acavity including: i) a first cavity portion having a first inlet alignedwith an axis of the first cavity portion and a first outlet provided ina circumferential outer wall of the first cavity portion; and, ii) asecond cavity portion having a second inlet aligned with an axis of thesecond cavity portion and a second outlet provided in a circumferentialouter wall of the second cavity portion; and, b) an impeller providedwithin the cavity, the impeller including: i) a first set of vanes beingprovided within the first cavity portion so as to define a first pumpthat provides at least partial left ventricular function; and, ii) asecond set of vanes being provided within the second cavity portion soas to define a second pump that provides at least partial rightventricular function; and, c) a drive for rotating the impeller in thecavity and wherein the first and second pumps define a respective flowpath through the pump, each flow path having a minimal cross-sectionalarea of at least 50 mm². 21) A heart pump including: a) a housingforming a cavity including: i) at least one inlet aligned with an axisof the cavity; and, ii) at least one outlet provided in acircumferential outer wall of the housing; b) an impeller providedwithin the cavity, the impeller including vanes for urging fluid fromthe inlet radially outwardly to the outlet; and, c) a drive for rotatingthe impeller in the cavity and wherein the pump includes at least oneof: i) improved outflow pressure sensitivity (OPS); ii) improved axialpressure sensitivity (APS); iii) reduced radial hydraulic forces; iv)emboli tolerance; and, v) improved passive flow balancing.